Younger relative cut by retired writer (6)
I believe the answer is:
nephew
'younger relative' is the definition.
(the son of a sibling)
'cut by retired writer' is the wordplay.
'cut' becomes 'hew' (hew can mean to chop or cut).
'by' means one lot of letters go next to another.
'retired writer' becomes 'nep' (I can't explain this - if you can you should believe this answer much more).
'hew' put after 'nep' is 'NEPHEW'.
(Other definitions for nephew that I've seen before include "Brother's/sister's son" , "Sibling's son" , "A young male relation" , "Family relative" , "Has a duty (to do something)" .)