Pick on (5)
I believe the answer is:
tease
'pick on' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tease that I've seen before include "Playfully provoke" , "Comb to remove tangles and open out the fibres" , "Irritate playfully" , "Comb or card (wool)" , "Gently mock" .)