Exceptional acceleration in luxury car with electric starter (4)
I believe the answer is:
rare
'exceptional' is the definition.
(I've seen this before)
'acceleration in luxury car with electric starter' is the wordplay.
'acceleration' becomes 'a' (symbol used in Physics e.g. f=ma).
'in' is an insertion indicator.
'luxury car' becomes 'rr' (short for Rolls Royce).
'with' is a charade indicator (letters next to each other).
'starter' indicates taking the first letters.
The initial letter of 'electric' is 'e'.
'a' placed into 'rr' is 'rar'.
'rar'+'e'='RARE'
(Other definitions for rare that I've seen before include "Not easily come by like undercooked meat" , "Scarcely seen" , "Might a four-leaf clover be so" , "like Ben Jonson, Jack Young thought" , "Very, very occasional" .)