Lad holding key for the trunk (4)
I believe the answer is:
body
'the trunk' is the definition.
('body' can be a synonym of 'trunk')
'lad holding key' is the wordplay.
'lad' becomes 'boy' (I've seen this before).
'holding' indicates putting letters inside.
'key' becomes 'd' (I've seen this in other clues**).
'boy' enclosing 'd' is 'BODY'.
'for' acts as a link.
(Other definitions for body that I've seen before include "Physical structure of an organism" , "Corpus" , "Person - corporation" , "welcomed in Madeira" , "Non-verbal communication?" .)