Unattractive scheme involving island (5)
I believe the answer is:
plain
'unattractive' is the definition.
(similar in meaning)
'scheme involving island' is the wordplay.
'scheme' becomes 'plan' (synonyms).
'involving' indicates putting letters inside.
'island' becomes 'i' (geographical abbreviation).
'plan' going around 'i' is 'PLAIN'.
(Other definitions for plain that I've seen before include "Unattractive - flat" , "Simple or apparent" , "Evident; not fancy" , "Undecorated; clear" , "Unprepossessing" .)