Comment concerning Matthew's successor (6)
I believe the answer is:
remark
'comment' is the definition.
(a remark is a spoken comment)
'concerning matthew's successor' is the wordplay.
'concerning' becomes 're' (both can mean on a particular topic).
'matthew's successor' becomes 'Mark' (as in the gospels Matthew, Mark, Luke and John).
're'+'mark'='REMARK'
(Other definitions for remark that I've seen before include "Say as a comment" , "Notice; happen to say" , "Make mention of" , "Something said; take notice of" , "Comment, observation" .)