Improperly, mainly invest in island (5)
I believe the answer is:
nevis
'island' is the definition.
(I've seen this before)
'improperly mainly invest' is the wordplay.
'improperly' indicates anagramming the letters (I've seen this in other clues).
'mainly' means to remove the last letter (virtually or mainly the word, but not all of it).
'invest' with its final letter removed is 'inves'.
'inves' is an anagram of 'NEVIS'.
'in' is the link.
(Other definitions for nevis that I've seen before include "Ben ..... is highest mountain in Britain" , "The principal ben" , "The Biggest Ben?" , "Ben -, highest Br. spot" , "Ben -, UK's highest mountain" .)