Relative setback for writer (hack) (6)
I believe the answer is:
nephew
'relative' is the definition.
(type of relative)
'setback for writer hack' is the wordplay.
'setback for' shows that the letters should be reversed in order (letters are set down in backwards order).
'writer' becomes 'pen' (something which writes).
'hack' becomes 'hew' (I've seen this before).
'pen' in reverse letter order is 'nep'.
'nep'+'hew'='NEPHEW'
(Other definitions for nephew that I've seen before include "Brother's/sister's son" , "A young male relation" , "A sibling's child" , "Sibling's son" , "Male relative" .)