Key? Perhaps. Hole? So we're told (5)

I believe the answer is:

islet

'key?' is the definition.
(both can mean a small island)

I cannot understand how the remainder of the clue works.

(Other definitions for islet that I've seen before include "Detached bit of land" , "One of Langerhans's" , "Eyot, key" , "Stile (anag)" , "Very small area surrounded by water" .)

I've seen this clue in The Everyman puzzle in The Observer.

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