Sets up as rival to a successful body-building business? (7,2)

'body-building business?' is the definition.
I don't know anything about this answer so I can't tell whether this works.

'sets up as rival to a successful' is the wordplay.
I cannot quite see how this works, but
'successful' could be 'in' ('in' can be similar in meaning to 'successful') and 'in' is found within the answer.
The remaining letters 'muscles' is a valid word which might be clued in a way I don't see.
This may be the basis of clue (or it may be nonsense).

Can you help me to learn more?