Fast car reversed inside - there's the rub (8)
I believe the answer is:
liniment
'the rub' is the definition.
(a liniment is an ointment rubbed on the body)
'fast car reversed inside' is the wordplay.
'fast' becomes 'lent' (Lent is a period of fasting in Christianity).
'car' becomes 'mini' (Mini Cooper).
'reversed' says the letters should be written in reverse.
'inside' means one lot of letters goes inside another.
'mini' written backwards gives 'inim'.
'lent' enclosing 'inim' is 'LINIMENT'.
'there's' acts as a link.
(Other definitions for liniment that I've seen before include "Lotion" , "Embrocation" , "Medicinal liquid for the muscles" , "Liquid for rubbing into skin" , "application" .)