Sea raid, act deployed to gain island (8)
I believe the answer is:
adriatic
'sea' is the definition.
(Adriatic is an example)
'raid act deployed to gain island' is the wordplay.
'deployed' is an anagram indicator.
'to gain' means one lot of letters goes inside another.
'island' becomes 'i' (geographical abbreviation).
'raid'+'act'='raidact'
'raidact' with letters rearranged gives 'adriatc'.
'adriatc' going around 'i' is 'ADRIATIC'.
(Other definitions for adriatic that I've seen before include "Sea between Italy and the Balkans" , "Sea seen from Bari" , "The drink" , "Sea between Italy and Slav republics" , "Venice was once known as Queen of this Sea" .)