Car, primarily an old banger, not costing a lot (5)
I believe the answer is:
cheap
'not costing a lot' is the definition.
(something cheap doesn't cost much)
'car primarily an old banger' is the wordplay.
'primarily' suggests taking the first letters.
'an old banger' becomes 'heap' (heap can mean an old car).
The first letter of 'car' is 'c'.
'c'+'heap'='CHEAP'
(Other definitions for cheap that I've seen before include "bargain" , "Low in price or tastelessly showy" , "Budget -cheesy" , "Contemptible" , "Base" .)