Mark indicating correctness (4)
I believe the answer is:
tick
'mark indicating correctness' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for tick that I've seen before include "Second" , "Parasite - mark of approval" , "Check the small mattress?" , "Blood-sucking mite or parasitic insect" , "Mark of work done (right)" .)