Grumble when vehicle has minimal power (4)
I believe the answer is:
carp
'grumble' is the definition.
'vehicle has minimal power' is the wordplay.
'vehicle' becomes 'car' (car is a kind of vehicle).
'has' says to put letters next to each other.
'minimal' says to take the initial letters.
The first letter of 'power' is 'p'.
'car'+'p'='CARP'
'when' is the link.
(Other definitions for carp that I've seen before include "Nagging fish?" , "Keep complaining" , "Fish < and beef" , "Large freshwater fish" , "Fish; nag pettily" .)