Annoys fishermen heartlessly (6)
I believe the answer is:
angers
'annoys' is the definition.
(to annoy is to anger)
'fishermen heartlessly' is the wordplay.
'fishermen' becomes 'anglers' (angler is a kind of fisherman).
'heartlessly' means to remove the middle letters.
'anglers' with its centre removed is 'ANGERS'.
(Other definitions for angers that I've seen before include "Irritates" , "Maine city" , "annoys" , "Infuriates" , "Maddens" .)