Goads son? That's gratuitous (8)
I believe the answer is:
needless
'gratuitous' is the definition.
('needless' can be a synonym of 'gratuitous')
'goads son?' is the wordplay.
'goads' becomes 'needles' ('needle' can be a synonym of 'goad').
'son?' becomes 's'.
'needles'+'s'='NEEDLESS'
'that's' acts as a link.
(Other definitions for needless that I've seen before include "Lessened, as not required" , "luxury" , "Otiose" , "uncalled for" , "Gratuitous" .)