After stunning success with exercise, knock back some cash in Moscow? (5)

'some cash in moscow?' is the definition.
I can't tell whether this defines the answer.

'after stunning success with exercise knock back' is the wordplay.
'after' says to put letters next to each other.
'stunning success' becomes 'ko' (I can't justify this - if you can you should believe this answer much more).
'with' says to put letters next to each other.
'exercise' becomes 'pe' (Physical Education).
'back' indicates one should take the final letters.
The final letter of 'knock' is 'k'.
'pe'+'k' is 'pek'.
'ko' put next to 'pek' is 'KOPEK'.

Can you help me to learn more?