Comment about gospel (6)
I believe the answer is:
remark
'comment' is the definition.
(a remark is a spoken comment)
'about gospel' is the wordplay.
'about' becomes 're' (regarding).
'gospel' becomes 'Mark' (book of the New Testament).
're'+'mark'='REMARK'
(Other definitions for remark that I've seen before include "(E.g. Virgil's) pastoral poems" , "Say as a comment" , "Mention - observe" , "Observation - notice" , "what someone says?" .)