Comment on Matthew's successor (6)
I believe the answer is:
remark
'comment' is the definition.
(a remark is a spoken comment)
'on matthew's successor' is the wordplay.
'on' becomes 're' ('on' can be mean 'with reference to').
'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 "Notice; happen to say" , "Comment or mention" , "Written or spoken comment" , "Make mention of" , "Mention - observe" .)