Rude writer's under a vehicle! (7)
I believe the answer is:
abusive
'rude' is the definition.
(I've seen this before)
'writer's under a vehicle' is the wordplay.
'writer's' becomes 'i've' ('I have' can refer to the person writing).
'under' is a charade indicator (letters next to each other).
'vehicle' becomes 'bus' (bus is a kind of vehicle).
'a'+'bus'='abus'
'ive' put after 'abus' is 'ABUSIVE'.
(Other definitions for abusive that I've seen before include "Vituperative" , "Insulting; brutal" , "Rude, insulting" , "Given to hurling insults" , "Swearing, insulting" .)