Ventilation aid in vehicle kept by former supporter (9,3)
I believe the answer is:
extractor fan
'ventilation aid' is the definition.
(device assisting with ventilation)
'vehicle kept by former supporter' is the wordplay.
'vehicle' becomes 'tractor' (tractor is a kind of vehicle).
'kept by' indicates putting letters inside.
'former' becomes 'ex' (prefix meaning former or onetime).
'supporter' becomes 'fan' (fan is a kind of supporter).
'ex'+'fan'='exfan'
'tractor' put inside 'exfan' is 'EXTRACTOR FAN'.
'in' is the link.
(Other definitions for extractor fan that I've seen before include "Device to remove stale air or fumes" , "Kitchen odour remover" .)