Principally trees' old roots support immense trunks (5)

I believe the answer is:

torsi

'trunks' is the definition.
('torso' can be a synonym of 'trunk')

'principally trees' old roots support immense' is the wordplay.
'principally' suggests taking the first letters.
The initial letters of 'trees old roots support immense' is 'TORSI'.

(Another definition for torsi that I've seen is " Rare parts of statues".)

I've seen this clue in The Telegraph.

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