Small room with first musician (7)
I believe the answer is:
cellist
'musician' is the definition.
(cellist is a kind of musician)
'small room with first' is the wordplay.
'small room' becomes 'cell' (I've seen this before).
'with' is a charade indicator (letters next to each other).
'first' becomes 'ist'.
'cell'+'ist'='CELLIST'
(Other definitions for cellist that I've seen before include "Member of orchestra" , "Du Pre or Lloyd Weber" , "Lloyd Webber?" , "String player" , "Musician in string section" .)