Not completely mature, but turning over a new leaf? (5)
I believe the answer is:
organ
'leaf?' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe there's an association between them I don't understand?
'not completely mature but turning over a new' is the wordplay.
'not completely mature but turning over' becomes 'org' (I can't justify this - if you can you should give a lot more credence to this answer).
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'org'+'a'+'n'='ORGAN'
Can you help me to learn more?
(Other definitions for organ that I've seen before include "Wind instrument found in the body." , "Church keyboard instrument" , "Possibly the heart" , "colon?" , "With prompt enthusiasm" .)