Fine young officer, on the quiet (4)
I believe the answer is:
posh
'fine' is the definition.
(thesaurus)
'young officer on the quiet' is the wordplay.
'young officer' becomes 'po' (I am not sure about this - if you are sure you should believe this answer much more).
'on' means one lot of letters go next to another.
'the quiet' becomes 'sh' (both can be said to tell someone to be quiet).
'po'+'sh'='POSH'
Can you help me to learn more?
(Other definitions for posh that I've seen before include "Shop around like upper class" , "Elegantly luxurious" , "Luxuriously stylish" , "Of the wealthy classes" , "Aristocratic" .)