Keep quiet about mum's accident (5)
I believe the answer is:
smash
'accident' is the definition.
(smash is a kind of accident)
'keep quiet about mum's' is the wordplay.
'keep quiet' becomes 'sh'.
'about' is an insertion indicator.
'mum's' becomes 'mas' ('ma' can be a synonym of 'mum').
'sh' going around 'mas' is 'SMASH'.
(Other definitions for smash that I've seen before include "Break noisely into pieces" , "Hard tennis shot" , "Ruin; big (success)" , "Break suddenly into little pieces" , "Wreck" .)