Room first for musician (7)
I believe the answer is:
cellist
'for musician' is the definition.
(cellist is a kind of musician)
'room first' is the wordplay.
'room' becomes 'cell' (cell is a kind of room).
'first' becomes 'ist' (ist resembles 1st).
'cell'+'ist'='CELLIST'
(Other definitions for cellist that I've seen before include "Lloyd Webber?" , "String player" , "Musician in string section" , "Member of orchestra" , "E.g. Casals, du Pre" .)