That's the key! (4)
I believe the answer is:
isle
'that's the key' is the definition.
(key can mean a small island)
This is all the clue.
(Other definitions for isle that I've seen before include "-- of Ely, Dogs, Man" , "Sea-girt area" , "This is sceptred!" , "One surrounded by water" , "Man, Wight, etc" .)