Widest track leading outside (8)
I believe the answer is:
broadest
'widest' is the definition.
('broad' can be a synonym of 'wide')
'track leading outside' is the wordplay.
'track' becomes 'road'.
'leading' becomes 'best'.
'outside' is an insertion indicator.
'road' placed within 'best' is 'BROADEST'.
(Other definitions for broadest that I've seen before include "Widest" , "Most general, extensive" , "Most unspecific" .)