Group of five hundred enthralled by young male (4)
I believe the answer is:
body
'group' is the definition.
(body is a kind of group)
'five hundred enthralled by young male' is the wordplay.
'five hundred' becomes 'd' ('D' can be a synonym of 'five hundred').
'enthralled by' means one lot of letters goes inside another ('enthral' can mean 'capture').
'young male' becomes 'boy' (I've seen this before).
'd' put into 'boy' is 'BODY'.
'of' is the link.
(Other definitions for body that I've seen before include "Physical structure of an organism" , "Full flavour in wine" , "Hare or Burke, for example" , "organisation" , "(Physical part of) person (or persons)" .)