Fishmonger possibly the only supplier? (4,5)

I believe the answer is:

sole agent

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'fishmonger possibly' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.

'only supplier?' is the wordplay.
'only' becomes 'sole' (synonyms).
'supplier?' becomes 'agent' (I can't explain this - if you can you should give a lot more credence to this answer).
'sole'+'agent'='SOLE AGENT'

'the' is the link.

Can you help me to learn more?

I've seen this clue in The Independent.

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