Fix and treat stain with care (9)
I believe the answer is:
ascertain
'fix' is the definition.
(thesaurus)
'treat stain with care' is the wordplay.
'treat' indicates an anagram.
'with' is a charade indicator (letters next to each other).
'stain' after 'care' is 'carestain'.
'carestain' with letters rearranged gives 'ASCERTAIN'.
'and' is the link.
(Other definitions for ascertain that I've seen before include "Confirm, make sure" , "Learn" , "Find out for sure, determine" , "Become aware" , "Find out - make sure" .)