Youth pocketing key for the trunk (4)
I believe the answer is:
body
'the trunk' is the definition.
('body' can be a synonym of 'trunk')
'youth pocketing key' is the wordplay.
'youth' becomes 'boy' (I've seen this before).
'pocketing' indicates putting letters inside.
'key' becomes 'd' (I've seen this in other clues**).
'boy' going around 'd' is 'BODY'.
'for' is the link.
(Other definitions for body that I've seen before include "Stiff" , "Person - corporation" , "organisation" , "Physique" , "One-piece undergarment for women, covering the torso" .)