Improve painting that’s thoroughly wet: what a pain (7)
I believe the answer is:
retouch
'improve' is the definition.
(I know that retouch is a more specific form of enhance)
'wet what a pain' is the wordplay.
'wet' becomes 'ret' (I've seen this in other clues).
'what a pain' becomes 'ouch' (I can't explain this - if you can you should give a lot more credence to this answer).
'ret'+'ouch'='RETOUCH'
'painting that's thoroughly' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for retouch that I've seen before include "Fine-tune" , "Improve a photo, with a pencil say" , "Make slight improvements to (a photo/picture)" , "Repair a painting" , "Paint over" .)