Ill-prepared monarch sheltered, having lost crown at sea (8)
I believe the answer is:
ethelred
'ill-prepared monarch' is the definition.
(Ethelred the Unready, medieval king)
'sheltered having lost crown at sea' is the wordplay.
'having lost crown' means to remove the first letter (crown can mean the top letter).
'at sea' indicates an anagram.
'sheltered' with its initial letter taken off is 'heltered'.
'heltered' is an anagram of 'ETHELRED'.
(Other definitions for ethelred that I've seen before include "Early English king" , "ancient King" , "Unready monarch" , "King of England" , "Alfred the Great's brother and predecessor, d. 871 -- the elder (anag)" .)