Peacekeepers on somewhat empty Shetland island (4)
I believe the answer is:
'shetland island' is the definition.
(an isle of the Shetland Islands)
'peacekeepers on somewhat empty' is the wordplay.
'peacekeepers' becomes 'un'.
'on' says to put letters next to each other.
'empty' means to remove the middle letters.
'somewhat' with its middle taken out is 'st'.
(Another definition for unst that I've seen is " Is".)