Old number? Some leave the room (5)
I believe the answer is:
ether
'old number?' is the definition.
(I've seen this in another clue)
'some leave the room' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'ETHER' can be found hidden inside 'leave the room'.
(Other definitions for ether that I've seen before include "Rare atmosphere" , "Imaginary interstellar medium" , "An early anaesthetic" , "Colourless volatile highly inflammable liquid" , "Volatile organic liquid once used as a general anaesthetic" .)