Artist's touch-up (3)
I believe the answer is:
dab
'artist's touch-up' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for dab that I've seen before include "Gently wipe" , "A bad, backward fish" , "Light stroke" , "[SOMETHING FISHY]" , "Apply light touches of colour to the fish" .)