Minor key used by hotel singers backing (5)
I believe the answer is:
islet
'minor key' is the definition.
(I've seen this in another clue)
'used by hotel singers backing' is the wordplay.
'used by' indicates a hidden word.
'backing' says the letters should be written in reverse.
'telsi' is hidden within 'hotel singers'.
'telsi' reversed gives 'ISLET'.
(Other definitions for islet that I've seen before include "Tiles to make a small island" , "Very small area surrounded by water" , "Piece of land in e.g. river" , "Detached territory" , "Cay, for example" .)