Mother Riley's initial ruin! (3)
I believe the answer is:
mar
'ruin' is the definition.
(I've seen this before)
'mother riley's initial' is the wordplay.
'mother' becomes 'ma' (ma is a kind of mother).
'initial' suggests taking the first letters.
The initial letter of 'riley' is 'r'.
'ma'+'r'='MAR'
(Other definitions for mar that I've seen before include "Ram into and spoil" , "Spoil stuff" , "Disfigure" , "Make imperfect" , "hurt" .)