Old bus set out? Certainly! (9)
I believe the answer is:
doubtless
'certainly' is the definition.
(I've seen this before)
'old bus set out?' is the wordplay.
'out?' indicates an anagram (I've seen 'out of' mean this).
'old'+'bus'+'set'='oldbusset'
'oldbusset' anagrammed gives 'DOUBTLESS'.
(Other definitions for doubtless that I've seen before include "Definitely not" , "Assuredly" , "Positive" , "Unquestionably" , "Probably, certainly" .)