Damage caused by one married couple (6)
I believe the answer is:
impair
'damage' is the definition.
(impairing is a kind of damaging)
'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'
'caused by' acts as a link.
(Other definitions for impair that I've seen before include "Hinder" , "Cause weakening of, damage" , "Hobble" , "'Mar, damage (6)'" , "Spoil strength, quality of" .)