Box tree, in part (5)

I believe the answer is:

trunk

I believe this is a double definition.

'box' is the first definition.
(thesaurus)

'tree in part' is the second definition.

(Other definitions for trunk that I've seen before include "Main stem of tree" , "Torso - box" , "main part of plane" , "boot of Cadillac?" , "Proboscis" .)

I've seen this clue in The Times.

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