Area with carbon in a vehicle's frame (6)
I believe the answer is:
abacus
'frame' is the definition.
(I have seen 'frame of sorts' mean 'abacus' so perhaps 'frame' could also mean 'abacus')
'area with carbon in a vehicle's' is the wordplay.
'area' becomes 'a' (maths abbreviation).
'with' is a charade indicator (letters next to each other).
'carbon' becomes 'C' (chemical symbol).
'in' is an insertion indicator.
'vehicle' becomes 'bus' (public transport vehicle).
'a'+'c'='ac'
'a'+'bus'='abus'
'ac' going within 'abus' is 'ABACUS'.
(Other definitions for abacus that I've seen before include "Counting machine" , "Early counting-frame" , "Old counter" , "Old calculator" , "Beaded counting frame" .)