The most common and most likely answer to this crossword clue is the 4 letter word MATE
I've seen this crossword clue in The Guardian
M
A
T
E
More clues leading to the result MATE
- Spymaster and worried colleague
- Male took food for friend
- Friend and Monsieur dined
- Partner in defeat on board
- Ally regularly embattled
- Partner declared winning position on board
- Sailor tucked in after end of storm
- Partner? One may be a fool's or a scholar's
- Worried to support married couple
- China‘s tea with pronounced difference
Buddy explains!

The straight part
We know that "Sailor/to finish the game on board" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sailor/to finish the game on board = mate.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 MATE mean?
MATE noun- A fellow, comrade, colleague, partner or someone with whom something is shared, e.g. shipmate, classmate.
- (especially of a non-human animal) A breeding partner.
- A friend, usually of the same sex.
- Friendly term of address to a stranger, usually male, of similar age
- In naval ranks, a non-commissioned officer or his subordinate (e.g. Boatswain's Mate, Gunner's Mate, Sailmaker's Mate, etc).
- A ship's officer, subordinate to the master on a commercial ship.
- A first mate.
- A technical assistant in certain trades (e.g. gasfitter's mate, plumber's mate); sometimes an apprentice.
- The other member of a matched pair of objects.
- A suitable companion; a match; an equal.
- To match, fit together without space between.
- To copulate.
- To pair in order to raise offspring
- To arrange in matched pairs.
- To introduce (animals) together for the purpose of breeding.
- (of an animal) To copulate with.
- To marry; to match (a person).
- To match oneself against; to oppose as equal; to compete with.
- To fit (objects) together without space between.
- To move (a space shuttle orbiter) onto the back of an aircraft that can carry it.
- The conclusive victory in a game of chess that occurs when an opponent's king is threatened with unavoidable capture.
- (by extension) Any losing situation with no escape; utter defeat.
- To put the king of an opponent into checkmate.
- (by extension) To place in a losing situation that has no escape.
- To confuse; to confound.
- The abovementioned plant; the leaves and shoots used for the tea
- An evergreen tree, Ilex paraquariensis, native to South America, cultivated for its leaves.
- A beverage, resembling tea, made from the dried leaves of this plant.
- A cup of this drink.