Comment on stain (6)
I believe the answer is:
remark
'comment' is the definition.
(a remark is a spoken comment)
'on stain' is the wordplay.
'on' becomes 're' ('on' can be mean 'with reference to').
'stain' becomes 'mark' (synonyms).
're'+'mark'='REMARK'
(Other definitions for remark that I've seen before include "Observation - notice" , "Written or spoken comment" , "Incidental observation" , "Something said; take notice of" , "Comment, observation" .)