Spoil one married couple (6)
I believe the answer is:
impair
'spoil' is the definition.
(synonyms)
'one married couple' is the wordplay.
'one' becomes 'i' (Roman numeral).
'married' becomes 'm' (abbreviation, e.g. in genealogy).
'couple' becomes 'pair' (synonyms).
'i'+'m'+'pair'='IMPAIR'
(Other definitions for impair that I've seen before include "Damage or handicap" , "'Mar, spoil (6)'" , "Harm" , "Cause weakening of, damage" , "Courteous man (colloq.)" .)