Harm setter’s mate (6)
I believe the answer is:
impair
'harm' is the definition.
(I've seen this before)
'setter's mate' is the wordplay.
'setter's' becomes 'im' (referring to the person who set the crossword).
'mate' becomes 'pair' (synonyms).
'im'+'pair'='IMPAIR'
(Other definitions for impair that I've seen before include "Courteous man (colloq.)" , "Make worse or less effective" , "Damage or handicap" , "blunt" , "Hurt or handicap" .)