Mother, get on ruin! (6)
I believe the answer is:
damage
'ruin' is the definition.
(thesaurus)
'mother get on' is the wordplay.
'mother' becomes 'dam' (female parent of an animal).
'get on' becomes 'age' (synonyms).
'dam'+'age'='DAMAGE'
(Other definitions for damage that I've seen before include "Detrimental effect" , "Physical harm" , "Deface, break" , "destruction" , "Harm, impair" .)