Provokes son, which is unnecessary (8)
I believe the answer is:
needless
'unnecessary' is the definition.
(similar in meaning)
'provokes son' is the wordplay.
'provokes' becomes 'needles' (needling is a kind of provoking).
'son' becomes 's'.
'needles'+'s'='NEEDLESS'
'which is' is the link.
(Other definitions for needless that I've seen before include "Surplus to requirements" , "Uncalled-for" , "Unnecessary and unwarranted" , "- - to say" , "without provocation?" .)