Success at twenty? (5)
I believe the answer is:
score
This clue is a double definition.
'success' is the first definition.
(score is a kind of success)
'twenty?' is the second definition.
('score' can be a synonym of 'twenty')
'at' acts as a link.
(Other definitions for score that I've seen before include "Deeply scratch" , "Tally; group of twenty" , "Notch; twenty" , "Gain a point (20 of them?)" , "Shoot a goal for a twenty" .)