Stiff young man hoarding diamonds (4)
I believe the answer is:
body
'stiff' is the definition.
(I've seen this before)
'young man hoarding diamonds' is the wordplay.
'young man' becomes 'boy' (a boy is a male child).
'hoarding' means one lot of letters goes inside another.
'diamonds' becomes 'd' (abbreviation for the card suit diamonds).
'boy' going around 'd' is 'BODY'.
(Other definitions for body that I've seen before include "Group of people" , "Substance" , "Hare or Burke, for example" , "Corpse; full quality of wine" , "Corpus" .)