Comment about stain (6)
I believe the answer is:
remark
'comment' is the definition.
('remark' can be a synonym of 'comment')
'about stain' is the wordplay.
'about' becomes 're' (regarding).
'stain' becomes 'mark' ('mark' can be a synonym of 'stain').
're'+'mark'='REMARK'
(Other definitions for remark that I've seen before include "Notice or comment on" , "what someone says?" , "Written or spoken comment" , "Something said; take notice of" , "Say as a comment" .)