Complain, parking vehicle in front (4)
I believe the answer is:
carp
'complain' is the definition.
(carp can informally mean to complain or whinge)
'parking vehicle in front' is the wordplay.
'parking' becomes 'p' (abbreviation on road signs).
'vehicle' becomes 'car' (car is a kind of vehicle).
'in front' means one lot of letters go next to another.
'p' put after 'car' is 'CARP'.
(Other definitions for carp that I've seen before include "ish" , "Nagging fish?" , "Go on at" , "Complain like goldfish" , "Swimmer" .)