Very inexpensive old car retained by commanding officer (6)
I believe the answer is:
Here is my best explanation:
'very inexpensive' is the definition.
'old car retained by commanding officer' is the subsidiary indication.
'old car' becomes 'heap' (informal term for a dilapidated vehicle).
'retained by' is an insertion indicator.
'commanding officer' becomes 'co'.
'heap' inserted 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", "not expensive", "Shoddy", "Inferior product", "of little value" and "of poor quality".)