Dispute when behind vehicle loudly reversed (6)
I believe the answer is:
fracas
'dispute' is the definition.
(fracas is a kind of dispute)
'when behind vehicle loudly reversed' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'behind' is a charade indicator (letters next to each other).
'vehicle' becomes 'car' (car is a kind of vehicle).
'loudly' becomes 'f' (short for forte - 'loud' in music).
'reversed' says the letters should be written in reverse.
'car'+'f'='carf'
'carf' reversed gives 'frac'.
'as' after 'frac' is 'FRACAS'.
(Other definitions for fracas that I've seen before include "A scarf (anag)" , "Fight" , "Scrap" , "Bust-up" , "Dispute" .)