Parking vehicle first, then complain (4)
I believe the answer is:
carp
'complain' is the definition.
(carp can informally mean to complain or whinge)
'parking vehicle first' is the wordplay.
'parking' becomes 'p' (abbreviation on road signs).
'vehicle' becomes 'car' (car is a kind of vehicle).
'first' means one lot of letters go next to another.
'p' put after 'car' is 'CARP'.
'then' acts as a link.
(Other definitions for carp that I've seen before include "Cavil - fish" , "Complain in manner of goldfish" , "Fish - complain" , "Fish; find fault continually" , "Fish often kept in ponds" .)