Most audacious and better than the rest — mature within (7)
I believe the answer is:
boldest
'most audacious' is the definition.
('bold' can be similar in meaning to 'audacious')
'better than the rest mature within' is the wordplay.
'better than the rest' becomes 'best'.
'mature' becomes 'old' (associated in meaning).
'within' is an insertion indicator.
'best' enclosing 'old' is 'BOLDEST'.
'and' is the link.
(Other definitions for boldest that I've seen before include "Most daring or naughty" , "most striking" .)