Any one young fellow out of hundreds (4)
I believe the answer is:
body
'any one' is the definition.
'body' can be an answer for 'one' (thesaurus). I am not sure about the 'any' bit.
'young fellow out of hundreds' is the wordplay.
'young fellow' becomes 'boy' (I can't explain this - if you can you should believe this answer much more).
'out of' means one lot of letters goes inside another.
'hundreds' becomes 'd' (I've seen this in other clues).
'boy' enclosing 'd' is 'BODY'.
Can you help me to learn more?
(Other definitions for body that I've seen before include "(Physical part of) person (or persons)" , "Group of people" , "Corpse; full flavour" , "Corpus" , "Non-verbal communication?" .)