Simple scheme to capture island (5)
I believe the answer is:
plain
'simple' is the definition.
(synonyms)
'scheme to capture island' is the wordplay.
'scheme' becomes 'plan' (synonyms).
'to capture' means one lot of letters goes inside another.
'island' becomes 'i' (geographical abbreviation).
'plan' going around 'i' is 'PLAIN'.
(Other definitions for plain that I've seen before include "Clear - open - grassland" , "Sheer; frank" , "Ordinary; flat region" , "Unprepossessing" , "Unattractive - flat" .)