Poem about one black hat (9)

I believe the answer is:

sunbonnet

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'poem' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.

'one black hat' is the wordplay.
'one black' becomes 'sun' (I can't explain this - if you can you should believe this answer much more).
'hat' becomes 'bonnet' (bonnet is a kind of hat).
'sun'+'bonnet'='SUNBONNET'

'about' is the link.

Can you help me to learn more?

I've seen this clue in The Independent.

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