Wrote off about constituency car (3-6)
I believe the answer is:
two-seater
'car' is the definition.
(two-seater is a kind of car)
'wrote off about constituency' is the subsidiary indication.
'off' indicates an anagram.
'about' is an insertion indicator.
'constituency' becomes 'seat'.
'wrote' with letters rearranged gives 'twoer'.
'twoer' placed around 'seat' is 'twoseater'.
(Other definitions for two-seater that I've seen before include "Small car" , "Sports car" .)