The most common and most likely answer to this crossword clue is the 4 letter word TICK
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More clues leading to the result TICK
Buddy explains!

The straight part
We know that "Mark/moment" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mark/moment = tick.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 TICK mean?
TICK noun- A tiny woodland arachnid of the suborder Ixodida.
- A relatively quiet but sharp sound generally made repeatedly by moving machinery.
- A mark on any scale of measurement; a unit of measurement.
- A jiffy (unit of time defined by basic timer frequency).
- A short period of time, particularly a second.
- A periodic increment of damage or healing caused by an ongoing status effect.
- A mark (✓) made to indicate agreement, correctness or acknowledgement.
- A bird seen (or heard) by a birdwatcher, for the first time that day, year, trip, etc., and thus added to a list of observed birds.
- The whinchat.
- To make a clicking noise similar to the movement of the hands in an analog clock.
- To make a tick or checkmark.
- To work or operate, especially mechanically.
- To strike gently; to pat.
- To add a bird to a list of birds that have been seen (or heard).
- Ticking.
- A sheet that wraps around a mattress; the cover of a mattress, containing the filling.
- Credit, trust.
- To go on trust, or credit.
- To give tick; to trust.
- (place names) A goat.