Quietly enter a post office, first shop in top group placed in proximity (8)

'quietly enter a' is the definition.
I can't judge whether this definition defines the answer.

'post office first shop in top group placed in proximity' is the wordplay.
'post office' becomes 'po' (abbreviation).
'first shop' becomes 's' (ast letter of 'shop').
'in' means one lot of letters goes inside another.
'top group placed in proximity' becomes 'aping' (I can't justify this - if you can you should believe this answer much more).
'po'+'s'='pos'
'pos' going into 'aping' is 'APPOSING'.

Can you help me to learn more?