Mum's mum admits defeat (5)
I believe the answer is:
smash
'defeat' is the definition.
(I've seen this before)
'mum's mum admits' is the wordplay.
'mum's' becomes 'ma's' (both can mean mother).
'mum' becomes 'sh' (both can mean to be quiet).
'admits' indicates putting letters inside.
'mas' inserted within 'sh' is 'SMASH'.
(Other definitions for smash that I've seen before include "Overhead tennis shot" , "something very popular" , "Break like a glass or cup" , "Wreck" , "Shatter violently" .)