Comment about second Gospel (6)
I believe the answer is:
remark
'comment' is the definition.
(a remark is a spoken comment)
'second gospel' is the wordplay.
'second' becomes 're' (prefix meaning second in eg review, re-education).
'gospel' becomes 'Mark' (book of the New Testament).
're'+'mark'='REMARK'
'about' is the link.
(Other definitions for remark that I've seen before include "what someone says?" , "Notice; happen to say" , "(E.g. Virgil's) pastoral poems" , "Notice or comment on" , "Mention - observe" .)