Proper to impound a car (4)
I believe the answer is:
fiat
'car' is the definition.
(car manufacturer)
'proper to impound a' is the wordplay.
'proper' becomes 'fit' (I've seen this in another clue).
'to impound' indicates putting letters inside.
'fit' going around 'a' is 'FIAT'.
(Other definitions for fiat that I've seen before include "Official sanction" , "mandate" , "Decree or official permission" , "Arbitrary order" , "car from Italy" .)