Trunk like this under bunk, upside down (5)
I believe the answer is:
torso
'trunk' is the definition.
('torso' can be a synonym of 'trunk')
'like this under bunk upside down' is the wordplay.
'like this' becomes 'so'.
'under' says to put letters next to each other.
'bunk' becomes 'rot' ('rot' can be a synonym of 'bunk').
'upside down' is a reversal indicator.
'rot' backwards is 'tor'.
'so' put after 'tor' is 'TORSO'.
(Other definitions for torso that I've seen before include "Poorly-dressed child" , "Trunk; headless, limbless body" , "Roots (anag)" , "Main part of human body" , "Body (excluding head, neck and limbs)" .)