Cheers when reversing car (4)
I believe the answer is:
fiat
'car' is the definition.
(car manufacturer)
'cheers when reversing' is the wordplay.
'cheers' becomes 'ta' (both can informally mean thank you).
'when' becomes 'if'.
'reversing' says the letters should be written in reverse.
'ta'+'if'='taif'
'taif' back-to-front is 'FIAT'.
(Other definitions for fiat that I've seen before include "Command" , "Arbitrary order" , "Order or decree - let it be done" , "Decree or official permission" , "Let it be done - it's a Latin order" .)