Killer in motor-car (4)
I believe the answer is:
orca
'killer' is the definition.
(synonyms)
'in motor-car' is the wordplay.
'in' indicates a hidden word.
'ORCA' is hidden within 'motor car'.
(Other definitions for orca that I've seen before include "Killer whales" , "Cor! a killer whale" , "grampus" , "Predatory black and white toothed whale" , "Killer in deep" .)