Harm married couple on island (6)
I believe the answer is:
impair
'harm' is the definition.
(I've seen this before)
'married couple on island' is the wordplay.
'married' becomes 'm' (abbreviation, e.g. in genealogy).
'couple' becomes 'pair' (synonyms).
'on' is a charade indicator (letters next to each other).
'island' becomes 'i' (geographical abbreviation).
'm'+'pair'='mpair'
'mpair' after 'i' is 'IMPAIR'.
(Other definitions for impair that I've seen before include "Hobble" , "'Mar, damage (6)'" , "Hurt or handicap" , "Damage or handicap" , "Spoil, degrade" .)