Certainly bound to change (2,5)
I believe the answer is:
no doubt
'certainly' is the definition.
(I've seen this before)
'bound to change' is the wordplay.
'change' is an anagram indicator.
'bound'+'to'='boundto'
'boundto' with letters rearranged gives 'NO DOUBT'.
(Other definitions for no doubt that I've seen before include "sign of agreement" , "Surely" , "Bound to, unquestionably" , "Almost certainly" , "To be sure" .)