The most common and most likely answer to this crossword clue is the 4 letter word SLOT
I've seen this crossword clue in The Guardian
S
L
O
T
More clues leading to the result SLOT
- Son meeting fate in designated period
- Scheduled time for opening
- It’s a deadly sin with husband leaving Gap
- Niche that’s followed by deerstalker
- Small fortune in hole
- Opening not taken, with second moving to first place
- … in place of endless idleness
- An opening possibly lost
- Opening line’s swamped by drunk
- Soak imbibing litre in bar…;
Buddy explains!

The straight part
We know that "Gap" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case gap = slot.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 SLOT mean?
SLOT noun- A broad, flat, wooden bar, a slat, especially as used to secure a door, window, etc.
- A metal bolt or wooden bar, especially as a crosspiece.
- An implement for baring, bolting, locking or securing a door, box, gate, lid, window or the like.
- A channel opening in the stator or rotor of a rotating machine for ventilation and insertion of windings.
- The barrel or tube of a wave.
- To bar, bolt or lock a door or window.
- To shut with violence; to slam.
- A narrow depression, perforation, or aperture; especially, one for the reception of a piece fitting or sliding in it.
- A gap in a schedule or sequence.
- The allocated time for an aircraft's departure or arrival at an airport's runway.
- In a flying display, the fourth position; after the leader and two wingmen.
- A space in memory or on disk etc. in which a particular type of object can be stored.
- A slot machine designed for gambling.
- The vagina.
- The track of an animal, especially a deer; spoor.
- (Antarctica) A crack or fissure in a glacier or snowfield; a chasm; a crevasse.
- To put something (such as a coin) into a slot (narrow aperture)
- To assign something or someone into a slot (gap in a schedule or sequence)
- To put something where it belongs.
- (Rhodesia, in the context of the Rhodesian Bush War) To kill.
- (Antarctica) To fall, or cause to fall, into a crevasse.