Improve drench that's painful (7)
I believe the answer is:
retouch
'improve' is the definition.
(thesaurus)
'drench that's painful' is the wordplay.
'drench' becomes 'ret' ('ret' can mean to soak something in water).
'that's painful' becomes 'ouch' (I've seen this before).
'ret'+'ouch'='RETOUCH'
(Other definitions for retouch that I've seen before include "Correct painting" , "Improve a photo, with a pencil say" , "Paint over" , "Make slight improvements to (a photo/picture)" , "Repair a painting" .)