Island beginning to erupt — heavens above! (4)
I believe the answer is:
skye
'island' is the definition.
(Scottish island)
'beginning to erupt heavens above' is the wordplay.
'beginning to' says to take the initial letters.
'heavens' becomes 'sky' (sky is a kind of heavens).
'above' is a charade indicator (letters next to each other) (in a down clue, some letters go above others).
The first letter of 'erupt' is 'e'.
'e' put after 'sky' is 'SKYE'.
(Other definitions for skye that I've seen before include "Home of the Cuillin mountains" , "Where Bonnie Prince Charlie fled after his defeat at Culloden" , "Destination of a princely boat trip" , "Scottish location" , "somewhere off Scotland's west coast" .)