To provoke second son is unnecessary (8)
I believe the answer is:
needless
'unnecessary' is the definition.
('needless' can be similar in meaning to 'unnecessary')
'to provoke second son' is the wordplay.
'to provoke' becomes 'needle' (needling is a kind of provoking**).
'second' becomes 's' (common abbreviation).
'son' becomes 's'.
'needle'+'s'+'s'='NEEDLESS'
'is' is the link.
(Other definitions for needless that I've seen before include "luxury" , "for no good reason" , "without provocation?" , "Unwanted" , "Unnecessary and unwarranted" .)