Agree with writer after bus goes back (9)
I believe the answer is:
subscribe
'agree with' is the definition.
(thesaurus)
'writer after bus goes back' is the wordplay.
'writer' becomes 'scribe' (scribe is a kind of writer).
'after' says to put letters next to each other.
'goes back' says the letters should be written backwards.
'bus' reversed gives 'sub'.
'scribe' put after 'sub' is 'SUBSCRIBE'.
(Other definitions for subscribe that I've seen before include "Give regular payments" , "Buy regularly" , "Put one's name down as a purchaser" , "Contribute money" , "Pay money regularly for, say, magazine" .)