Silly posh idiot first to oppress worker, one in sales (4,9)
I believe the answer is:
shop assistant
'one in sales' is the definition.
'silly posh idiot first to oppress worker' is the wordplay.
'silly' is an anagram indicator.
'idiot first' becomes 'assist' (I can't justify this - if you can you should believe this answer much more).
'to oppress' is a charade indicator (letters next to each other) (in a down clue, oppress can mean goes on top of).
'worker' becomes 'ant' (worker is a type of ant).
'posh' with letters rearranged gives 'shop'.
'shop'+'assist'+'ant'='SHOP-ASSISTANT'
Can you help me to learn more?
(Other definitions for shop assistant that I've seen before include "one employed in store" , "Salesperson" , "Worker in retail trade" , "Give grass to aid one" , "who should be helping customers?" .)