Silent in action, apprehending nasty car thief (9)
I believe the answer is:
larcenist
'thief' is the definition.
(larcenist is a kind of thief)
'silent in action apprehending nasty car' is the wordplay.
'in action' is an anagram indicator (I've seen this in other clues).
'apprehending' is an insertion indicator.
'nasty' is an anagram indicator (a nasty or wrong spelling).
'silent' with letters rearranged gives 'lenist'.
'car' anagrammed gives 'arc'.
'lenist' going around 'arc' is 'LARCENIST'.
(Other definitions for larcenist that I've seen before include "One criminal" , "Person who takes" , "One takes" , "Dishonest person" , "nicker" .)