Sort out one's trunks (5)

I believe the answer is:

torsi

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

'sort out one's' is the wordplay.
'out' is an anagram indicator (out can mean wrong or inaccurate).
'one' becomes 'i' (Roman numeral).
'sort' with letters rearranged gives 'tors'.
'tors'+'i'='TORSI'

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

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