Oldest bus around, certainly ... (9)
I believe the answer is:
doubtless
'certainly' is the definition.
(thesaurus)
'oldest bus around' is the wordplay.
'around' indicates an anagram.
'oldest'+'bus'='oldestbus'
'oldestbus' with letters rearranged gives 'DOUBTLESS'.
(Other definitions for doubtless that I've seen before include "Assuredly" , "Unquestionably" , "of course" , "Probably, certainly" , "Positive" .)