Following onset of illness, married couple weaken (6)
I believe the answer is:
impair
'weaken' is the definition.
(I've seen this in another clue)
'following onset of illness married couple' is the wordplay.
'following' means one lot of letters go next to another.
'onset of' suggests taking the first letters.
'couple' becomes 'pair' (synonyms).
The initial letters of 'illness married' is 'im'.
'im'+'pair' is 'IMPAIR'.
(Other definitions for impair that I've seen before include "'Mar, damage (6)'" , "Hobble" , "Spoil strength, quality of" , "Spoil, degrade" , "Damage or handicap" .)