Drive after parking vehicle used by mum (4)
I believe the answer is:
pram
'vehicle used by mum' is the definition.
(a pram for pushing a baby)
'drive after parking' is the wordplay.
'drive' becomes 'ram' (RAM drive on a computer).
'after' says to put letters next to each other.
'parking' becomes 'p'.
'ram' after 'p' is 'PRAM'.
(Other definitions for pram that I've seen before include "Nanny may use this" , "lighter" , "Baby transport" , "Baby's carriage" , "Ramp for a baby buggy" .)