Can tire badly, to be sure (7)
I believe the answer is:
certain
'sure' is the definition.
('certain' can be similar in meaning to 'sure')
'can tire badly' is the wordplay.
'badly' indicates anagramming the letters.
'can'+'tire'='cantire'
'cantire' with letters rearranged gives 'CERTAIN'.
'to be' is the link.
(Other definitions for certain that I've seen before include "Nice art (anag)" , "Quite sure, positive" , "Sure, definite" , "Absolutely sure" , "Known beyond doubt" .)