Room next to first musician (7)
I believe the answer is:
cellist
'musician' is the definition.
(I know that cellist is a type of musician)
'room next to first' is the wordplay.
'room' becomes 'cell' (cell is a kind of room).
'next to' means one lot of letters go next to another.
'first' becomes 'ist' (ist resembles cst).
'cell'+'ist'='CELLIST'
(Other definitions for cellist that I've seen before include "E.g. Casals, du Pre" , "Du Pre or Lloyd Weber" , "Member of orchestra" , "String player" , "Lloyd Webber?" .)