The most common and most likely answer to this crossword clue is the 2 letter word GO
I've seen this crossword clue in The Times
G
O
More clues leading to the result GO
- Runner's been and got lost, accepting contrary directions
- Energy and skill embodied by man who's pre-eminent in motor sport
- Indian state's leader is ambitious
- Vehicle certainly requiring good skill to corner?
- Sailor bathed in light as form of industrial action
- Fine skills must underpin good racing vehicles
- Racer has green light and skill to impress thousands
- Racing vehicle in middle of night with adequate skill
- Cargo boat finally transported small racing vehicle
- Minor worker, from the bottom, perhaps painting vehicle
Buddy explains!

The straight part
We know that "Runner" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case runner = go.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does GO mean?
GO noun- The act of going.
- A turn at something, or in something (e.g. a game).
- An attempt, a try.
- An approval or permission to do something, or that which has been approved.
- An act; the working or operation.
- A circumstance or occurrence; an incident, often unexpected.
- The fashion or mode.
- Noisy merriment.
- A glass of spirits; a quantity of spirits.
- Power of going or doing; energy; vitality; perseverance.
- The situation where a player cannot play a card which will not carry the aggregate count above thirty-one.
- A period of activity.
- (British slang) A dandy; a fashionable person.
- To move:
- (chiefly of a machine) To work or function (properly); to move or perform (as required).
- To start; to begin (an action or process).
- To take a turn, especially in a game.
- To attend.
- To proceed:
- To follow or travel along (a path):
- To extend (from one point in time or space to another).
- To lead (to a place); to give access to.
- To become. (The adjective that follows usually describes a negative state.)
- To assume the obligation or function of; to be, to serve as.
- To continuously or habitually be in a state.
- To come to (a certain condition or state).
- To change (from one value to another) in the meaning of wend.
- To turn out, to result; to come to (a certain result).
- To tend (toward a result).
- To contribute to a (specified) end product or result.
- To pass, to be used up:
- To die.
- To be discarded.
- To be lost or out:
- To break down or apart:
- To be sold.
- To be given, especially to be assigned or allotted.
- To survive or get by; to last or persist for a stated length of time.
- To have a certain record.
- To be authoritative, accepted, or valid:
- To say (something), to make a sound:
- To be expressed or composed (a certain way).
- To resort (to).
- To apply or subject oneself to:
- To fit (in a place, or together with something):
- To date.
- To attack:
- To be in general; to be usually.
- To take (a particular part or share); to participate in to the extent of.
- To yield or weigh.
- To offer, bid or bet an amount; to pay.
- To enjoy. (Compare go for.)
- To urinate or defecate.
- Working correctly and ready to commence operation; approved and able to be put into action.
- A strategic board game, originally from China, in which two players (black and white) attempt to control the largest area of the board with their counters.