Harass with pin (6)
I believe the answer is:
needle
I believe this clue is a double definition.
'harass' is the first definition.
(needling is a kind of harassing)
'pin' is the second definition.
(I've seen this before)
'with' acts as a link.
(Other definitions for needle that I've seen before include "Pointer - irritate" , "Annoy - syringe" , "Annoy by continuous criticism; stylus" , "point" , "Knitting rod" .)