Keep quiet about mum's contribution to an 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' (as in shushing someone).
'about' means one lot of letters goes inside another.
'mum's' becomes 'ma's' (ma can mean mother).
'sh' enclosing 'mas' is 'SMASH'.
'contribution to an' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for smash that I've seen before include "tremendous success!" , "Break like a glass or cup" , "Ruin; big (success)" , "something very popular" , "Break noisely into pieces" .)