One married couple weaken (6)
I believe the answer is:
impair
'weaken' is the definition.
(I've seen this in another clue)
'one married couple' is the wordplay.
'one' becomes 'I' (Roman numeral).
'married' becomes 'm'.
'couple' becomes 'pair' (synonyms).
'i'+'m'+'pair'='IMPAIR'
(Other definitions for impair that I've seen before include "Harm" , "Damage or handicap" , "Weaken, damage" , "Spoil strength, quality of" , "Spoil, degrade" .)