Key in a chain (4)
I believe the answer is:
isle
'key in a chain' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for isle that I've seen before include "This is sceptred!" , "Skye, for instance" , "Piece of land in water" , "Small area of land in water" , "---- of Dogs, Thames peninsula" .)