Mum's quiet after son in car accident (5)
I believe the answer is:
smash
'car accident' is the definition.
(smash can mean a road accident)
'mum's quiet after son' is the wordplay.
'mum' becomes 'ma' (ma can mean mother).
'quiet' becomes 'sh' (both can be said to tell someone to be quiet).
'after' says to put letters next to each other.
'son' becomes 's' (genealogical abbreviation for son).
'ma'+'sh'='mash'
'mash' after 's' is 'SMASH'.
'in' acts as a link.
(Other definitions for smash that I've seen before include "Break noisely into pieces" , "Overhead tennis shot" , "Overarm volley" , "Wreck" , "Break like a glass or cup" .)