A marked sign of approval (4)
I believe the answer is:
tick
'sign of approval' is the definition.
(I've seen this before)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for tick that I've seen before include "Mark of work done (right)" , "Make noise like old clock" , "Credit; mark as correct" , "Parasitic insect or clicking sound" , "which is irritating!" .)