Rising corporation poaching American banker's design (3,3)

'design' is the definition.
(I know that map out is a more specific form of the action design)

'rising corporation poaching american banker's' is the wordplay.
'rising' says the letters should be written backwards (in a down clue, letters go up).
'corporation' becomes 'tum' ('corporation' can humorously mean belly fat).
'poaching' means one lot of letters goes inside another.
'american banker's' becomes 'opa' (I can't explain this - if you can you should believe this answer much more).
'tum' enclosing 'opa' is 'tuopam'.
'tuopam' reversed gives 'MAP OUT'.

Can you help me to learn more?