Touch lightly; flatfish (3)
I believe the answer is:
dab
This clue is a double definition.
'touch lightly' is the first definition.
(I've seen this in another clue)
'flatfish' is the second definition.
(I've seen this in another clue)
(Other definitions for dab that I've seen before include "A bad, backward fish" , "Fingerprint - fish" , "Expert; flatfish" , "A bad sort of little fish" , "Apply light touches of colour to the fish" .)