Sooner? Yes indeed! (6)
I believe the answer is:
rather
This clue is a triple definition!
'sooner?' is the first definition.
('I'd sooner do this' means 'I'd rather do this')
'yes' is the second definition.
(thesaurus)
'indeed' is the third definition.
(thesaurus)
(Other definitions for rather that I've seen before include "with more justification" , "To a limited degree" , "A bit" , "Quite, sort of" , "On the contrary; sooner" .)