Vehicle parking? Grumble (4)
I believe the answer is:
carp
'grumble' is the definition.
(I've seen this in another clue)
'vehicle parking?' is the wordplay.
'vehicle' becomes 'car' (car is a kind of vehicle).
'parking?' becomes 'p' (abbreviation on road signs).
'car'+'p'='CARP'
(Other definitions for carp that I've seen before include "Fish often kept in ponds" , "Large freshwater fish" , "Fish; find fault continually" , "Fish - complain" , "Fish; nag pettily" .)