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' says to put letters next to each other (in a down clue, letters appear on others).
'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" .)