Charlie not only child in part of car (7)
I believe the answer is:
chassis
'part of car' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Perhaps there's an association between them I don't understand?
'charlie not only child in' is the wordplay.
'charlie' becomes 'ass' (both can mean a foolish person).
'not only child' becomes 'chis' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in' indicates putting letters inside.
'ass' inserted within 'chis' is 'CHASSIS'.
Can you help me to learn more?
(Other definitions for chassis that I've seen before include "Vehicle structure" , "Vehicle base frame" , "Car body" , "Undercarriage" , "Car's structural framework" .)