Ned went after a ‘Abstainer A-one’ which was successfully achieved (8)
I believe the answer is:
attained
'achieved' is the definition.
('attain' can be a synonym of 'achieve')
'ned went after a abstainer a-one which was successfully' is the wordplay.
I cannot quite see how this works, but
'a' is within the answer.
'abstainer' could be 'tt' (I've seen this in other clues) and 'tt' is located in the answer.
'one' could be 'i' (Roman numeral) and 'i' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for attained that I've seen before include "Reached (eg, age)" , "Reached a goal" , "Reached with effort" , "Accomplished, achieved" .)