This setter's virtually undoubted to get better! (7)
I believe the answer is:
improve
'get better' is the definition.
(I've seen this before)
'this setter's virtually undoubted' is the wordplay.
'this setter's' becomes 'im' (referring to the person who set the crossword).
'virtually undoubted' becomes 'prove' (I can't justify this - if you can you should give a lot more credence to this answer).
'im'+'prove'='IMPROVE'
'to' is the link.
Can you help me to learn more?
(Other definitions for improve that I've seen before include "Enhance" , "pick up" , "Make progress" , "Ameliorate" , "Polish" .)