Team guide, son of the mailman? (7)

I believe the answer is:

postboy

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'the mailman?' is the definition.
Both the answer and definition are singular nouns.
Maybe there's an association between them I don't understand?

'team guide son' is the wordplay.
'team guide' becomes 'post' (I can't justify this - if you can you should give a lot more credence to this answer).
'son' becomes 'boy' (synonyms).
'post'+'boy'='POSTBOY'

'of' acts as a link.

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