Decline following mother's ruin (6)
I believe the answer is:
damage
'ruin' is the definition.
(thesaurus)
'decline following mother's' is the wordplay.
'decline' becomes 'age' (I've seen this in other clues).
'following' means one lot of letters go next to another.
'mother' becomes 'dam' (female parent of an animal).
'age' after 'dam' is 'DAMAGE'.
(Other definitions for damage that I've seen before include "Deface, harm" , "Ravages" , "vandalism?" , "Mar" , "Physical harm" .)