Soldier captures island in ocean (4)
I believe the answer is:
main
'ocean' is the definition.
(synonyms)
'soldier captures island' is the wordplay.
'soldier' becomes 'man' (man can mean a soldier).
'captures' indicates putting letters inside.
'island' becomes 'i' (geographical abbreviation).
'man' enclosing 'i' is 'MAIN'.
'in' acts as a link.
(Other definitions for main that I've seen before include "Cardinal" , "location for 9 [ARMADA]" , "Ocean or principal pipe" , "Foremost" , "Most important or greatest in size" .)