Accomplishing anti-anti-G, possibly (9)
I believe the answer is:
attaining
'accomplishing' is the definition.
('attain' can be a synonym of 'accomplish')
'anti-anti-g possibly' is the wordplay.
'possibly' indicates an anagram.
'anti'+'anti'+'g'='antiantig'
'antiantig' anagrammed gives 'ATTAINING'.
(Other definitions for attaining that I've seen before include "Reaching" , "Accomplishing, achieving" .)