He's not yet an officer, and mostly no gentleman (5)
I believe the answer is:
'he's not yet an' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'officer and mostly no gentleman' is the wordplay.
'officer' becomes 'det' (I can't justify this - if you can you should believe this answer much more).
'and' is a charade indicator (letters next to each other).
'mostly' means to remove the last letter.
'no gentleman' becomes 'cad' (I've seen this in another clue).
'cad' with its final letter taken off is 'ca'.
'det' after 'ca' is 'CADET'.
Can you help me to learn more?
(Other definitions for cadet that I've seen before include "Police force trainee" , "recruit" , "Younger son's branch of a family" , "Armed services trainee" , "A trainee army officer" .)