Sharp, altering car identification (5)
I believe the answer is:
acrid
'sharp' is the definition.
(I've seen this before)
'altering car identification' is the wordplay.
'altering' indicates an anagram.
'identification' becomes 'ID' (abbreviation).
'car' with letters rearranged gives 'acr'.
'acr'+'id'='ACRID'
(Other definitions for acrid that I've seen before include "Stinging" , "Sarcastic" , "Astringent" , "Sharp - biting" , "causing corrosion" .)