Agreement nothing but false, certainly (2,5)
I believe the answer is:
no doubt
'certainly' is the definition.
(I've seen this before)
'agreement nothing but false' is the wordplay.
'agreement' becomes 'nod' (a nod indicates agreement).
'nothing' becomes 'O' (looks like zero - 0).
'false' is an anagram indicator.
'but' with letters rearranged gives 'ubt'.
'nod'+'o'+'ubt'='NO DOUBT'
(Other definitions for no doubt that I've seen before include "sign of agreement" , "Almost certainly" , "Surely" , "Bound to, unquestionably" , "To be sure" .)