A marked sign of approval (4)

I believe the answer is:

tick

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'sign of approval' is the definition.
(I've seen this before)

I cannot understand how the remainder of the clue works.

(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!" .)

I've seen this clue in the Evening Standard.

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