Crossword Genius

Key passage that has no opening note (4)

Ross

I believe the answer is:

isle

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

I don't understand the remainder of the clue.

(Other definitions for isle that I've seen before include "Man's one" , "Small area of land in water" , "eg Thanet" , "eg Capri" , "Sea-girt area" .)

I've seen this clue in The Independent.
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