Harsh critic in debate “discontented” with vehicle (9)
I believe the answer is:
detractor
'harsh critic' is the definition.
(a detractor is a critic or opponent of something)
'debate discontented with vehicle' is the wordplay.
'discontented' means to remove the middle letters (the word's contents are removed).
'with' is a charade indicator (letters next to each other).
'vehicle' becomes 'tractor' (tractor is a kind of vehicle).
'debate' with its centre removed is 'de'.
'de'+'tractor'='DETRACTOR'
'in' acts as a link.
(Other definitions for detractor that I've seen before include "one responsible for mud flying around" , "I make uncomplimentary remarks" , "Slighter" , "One disparages or belittles" , "one runs people down" .)