Upset writer cut sibling’s son (6)
I believe the answer is:
nephew
'sibling's son' is the definition.
(child of a sibling)
'upset writer cut' is the wordplay.
'upset' shows that the letters should be reversed in order.
'writer' becomes 'pen' (something which writes).
'cut' becomes 'hew' (hew can mean to chop or cut).
'pen' reversed gives 'nep'.
'nep'+'hew'='NEPHEW'
(Other definitions for nephew that I've seen before include "Edward V to Richard III relation" , "Male relative" , "Has a duty (to do something)" , "Sister's son" , "Family relative" .)