I'm returning posh car -- or part of car (6)
I believe the answer is:
mirror
'part of car' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'i'm returning posh car or' is the wordplay.
'returning' says the letters should be written in reverse.
'posh car' becomes 'rr' (short for Rolls Royce).
'im' backwards is 'mi'.
'mi'+'rr'+'or'='MIRROR'
Can you help me to learn more?
(Other definitions for mirror that I've seen before include "Image reflector" , "Glass - paper" , "Glass for reflection" , "Reflecting surface" , "Common device for reflection" .)