Young ones mostly fat (5)
I believe the answer is:
issue
'young' is the definition.
(thesaurus)
'ones mostly fat' is the wordplay.
'ones' becomes 'is' ('I' can be a synonym of 'one').
'mostly' means to remove the last letter.
'fat' becomes 'suet' (suet is a kind of fat).
'suet' with its final letter removed is 'sue'.
'is'+'sue'='ISSUE'
(Other definitions for issue that I've seen before include "Matter of dispute" , "Major topic for debate" , "Point in dispute" , "Children; topic for debate" , "Put out; result" .)