Eldest trees split across limb's base (9)

I believe the answer is:

firstborn

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'eldest' is the definition.
(synonyms)

'trees split across limb's base' is the wordplay.
I cannot quite understand how this works, but
'trees' could be 'firs' (fir is a kind of tree) and 'firs' is found within the answer.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

(Another definition for firstborn that I've seen is " Eldest child".)

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

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