Protection of key island beset by rising waters (5)
I believe the answer is:
aegis
'protection' is the definition.
(aegis can mean someone's guidance or protection)
'key island beset by rising waters' is the wordplay.
'key' becomes 'g' (musical key).
'island' becomes 'i' (geographical abbreviation).
'beset by' means one lot of letters goes inside another ('beset' could historically mean 'cover').
'rising' is a reversal indicator (in a down clue, letters go up).
'waters' becomes 'sea' (the sea is water).
'sea' written backwards gives 'aes'.
'g'+'i'='gi'
'gi' inserted into 'aes' is 'AEGIS'.
'of' acts as a link.
(Other definitions for aegis that I've seen before include "Auspices" , "Backing or support" , "Protection (of someone)" , "Protection or sponsorship" , "Protection - guardianship" .)