Harm one married couple (6)
I believe the answer is:
impair
'harm' is the definition.
(thesaurus)
'one married couple' is the wordplay.
'one' becomes 'i' (Roman numeral).
'married' becomes 'm'.
'couple' becomes 'pair' ('pair' can be a synonym of 'couple').
'i'+'m'+'pair'='IMPAIR'
(Other definitions for impair that I've seen before include "Spoil strength, quality of" , "Spoil, degrade" , "blunt" , "'Mar, spoil (6)'" , "Harm" .)