Reversing car in fast shows application (8)
I believe the answer is:
liniment
'application' is the definition.
(liniment is a kind of application)
'reversing car in fast' is the wordplay.
'reversing' is a reversal indicator.
'car' becomes 'mini' (Mini Cooper**).
'in' is an insertion indicator.
'fast' becomes 'lent' (I've seen this before).
'mini' written backwards gives 'inim'.
'inim' placed into 'lent' is 'LINIMENT'.
'shows' acts as a link.
(Other definitions for liniment that I've seen before include "Liquid for rubbing into skin" , "Lotion" , "Embrocation" , "Medicinal liquid for the muscles" , "application" .)