Small room with first musician (7)
I believe the answer is:
Here is my best explanation:
'musician' is the definition.
(cellist is a kind of musician)
'small room with first' is the subsidiary indication.
'small room' becomes 'cell' (I've seen this before).
'with' means one lot of letters go next to another.
'first' becomes 'ist'.
(Other definitions for cellist that I've seen before include "musician", "E.g. Casals, du Pre", "Member of orchestra", "Du Pre or Lloyd Weber", "Lloyd Webber?", "member", "Musician in string section" and "String player".)