Certainly nothing to chatter about (4)
I believe the answer is:
okay
'certainly' is the definition.
(thesaurus)
'nothing to chatter about' is the wordplay.
'nothing' becomes 'O' (O resembles 0).
'to chatter' becomes 'yak' (synonyms).
'about' says the letters should be written in reverse.
'yak' back-to-front is 'kay'.
'o'+'kay'='OKAY'
(Other definitions for okay that I've seen before include "Informally, all right" , "Yes; satisfactory" , "Leave" , "Alright" , "Give approval to" .)