American tree by which nursery rhyme is started (7)

I believe the answer is:

hickory

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'american tree' is the definition.
'hickory' can be an answer for 'tree' (hickory tree is a kind of tree). I'm not certain of the 'american' bit.

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for hickory that I've seen before include "Hard wood" , "Heavy wood" , "Nut tree" , "North American tree" .)

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