Small room on street accommodating one musician (7)
I believe the answer is:
Here is my best explanation:
'musician' is the definition.
(cellist is a kind of musician)
'small room on street accommodating one' is the subsidiary indication.
'small room' becomes 'cell' (I've seen this before).
'on street' becomes 'st' (I've seen this in other clues).
'accommodating' indicates putting letters inside.
'one' becomes 'i' (roman numeral).
'cellst' placed around 'i' is 'cellist'.
(Other definitions for cellist that I've seen before include "Musician in string section", "E.g. Casals, du Pre", "musician", "Member of orchestra", "String player", "Du Pre or Lloyd Weber", "Lloyd Webber?" and "member".)