Assured expert is parting sea (8)
I believe the answer is:
promised
'assured' is the definition.
('promise' can be a synonym of 'assure')
'expert is parting sea' is the wordplay.
'expert' becomes 'pro' (short for professional).
'parting' is an insertion indicator (inserted letters split or part another word).
'sea' becomes 'med' (short for Mediterranean).
'is' inserted within 'med' is 'mised'.
'pro'+'mised'='PROMISED'
(Other definitions for promised that I've seen before include "Betrothed" , "Undertook (to do something)" , "Made a commitment to do" , "undertaken" , "swore" .)