Very inexpensive old car retained by commanding officer (6)
I believe the answer is:
cheapo
'very inexpensive' is the definition.
'old car retained by commanding officer' is the wordplay.
'old car' becomes 'heap' (informal term for a dilapidated vehicle).
'retained by' means one lot of letters goes inside another.
'commanding officer' becomes 'co'.
'heap' put within 'co' is 'CHEAPO'.
(Other definitions for cheapo that I've seen before include "inexpensive and not much good" , "little outlay" , "Mickey Mouse" , "at rock bottom price" , "Extremely inexpensive" .)