Firm poo trouble? (8,7)

I believe the answer is:

business concern

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'trouble?' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.

'firm poo' is the wordplay.
'firm' becomes 'business' (synonyms).
'poo' becomes 'concern' (I can't justify this - if you can you should believe this answer much more).
'business'+'concern'='BUSINESS CONCERN'

Can you help me to learn more?

I've seen this clue in the Sydney Morning Herald.

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