The setter's test is better (7)
I believe the answer is:
improve
'better' is the definition.
('improve' can be a synonym of 'better')
'the setter's test' is the wordplay.
'the setter's' becomes 'im' (I've seen this in other clues).
'test' becomes 'prove' ('prove' can be a synonym of 'test').
'im'+'prove'='IMPROVE'
'is' is the link.
(Other definitions for improve that I've seen before include "Become better" , "Enhance" , "Change for the better" , "Make better in quality" , "Ameliorate" .)