Complain about vehicle parking (4)
I believe the answer is:
carp
'complain' is the definition.
(carp can informally mean to complain or whinge)
'vehicle parking' is the wordplay.
'vehicle' becomes 'car' (car is a kind of vehicle).
'parking' becomes 'p' (abbreviation on road signs).
'car'+'p'='CARP'
'about' acts as a link.
(Other definitions for carp that I've seen before include "Raise petty objections about goldfish" , "Find fault with a freshwater fish" , "Swimmer" , "Complain like goldfish" , "Complain continually" .)