Key to a room in paradise (4)
I believe the answer is:
eden
'in paradise' is the definition.
(I know that Eden is part of paradise)
'key to a room' is the wordplay.
'key' becomes 'E' (musical key).
'to a room' becomes 'den' (den is a kind of room).
'e'+'den'='EDEN'
(Other definitions for eden that I've seen before include "Early garden - Cornish project" , "River - garden" , "The primal garden" , "Foreign Secretary" , "Cornish garden; paradise" .)