Key (4)
'key' is the definition.
(I have seen 'Musical key' mean 'clef' so perhaps 'key' could also mean 'clef')
This is all the clue.
(Other definitions for clef that I've seen before include "Sign at start of stave" , "Staff note" , "Symbol written on sheet music to indicate pitch" , "Musical pitch indicator" , "Sign of (musical) pitch" .)
(Other definitions for isle that I've seen before include "Dogs, maybe" , "Such as Man or Bute" , "Small landmass" , "Could be Man" , "See 10 Down" .)
(Other definitions for hash that I've seen before include "Recooked dish for shah" , "Stew - key" , "What signifies a number" , "Jumble or mixture for shah" , "Complete mess" .)
'key' is the definition.
(both can mean a small island)
This is all the clue.
(Other definitions for islet that I've seen before include "Stile (anag)" , "Eyot, key" , "Little bit of land" , "Very small area surrounded by water" , "Ait or key" .)