In Massachusetts, "key" is an island (5)
I believe the answer is:
malta
'an island' is the definition.
(Malta is an example)
'in massachusetts key' is the wordplay.
'in' is an insertion indicator.
'massachusetts' becomes 'ma'.
'key' becomes 'alt' (I've seen this in other clues).
'ma' placed around 'alt' is 'MALTA'.
'is' acts as a link.
(Other definitions for malta that I've seen before include "small island" , "Island member of the European Union" , "Island European country" , "defiant island in WW2" , "Mediterranean island state" .)