Mark as correct (4)
I believe the answer is:
tick
'mark as correct' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for tick that I've seen before include "Blood-sucking arachnid" , "Mark correct, parasite" , "Minute" , "Check-mark or blood-sucking mite" , "Insect; noise of watch" .)