Group of four musicians (7)
I believe the answer is:
quartet
'group of four musicians' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for quartet that I've seen before include "Group of four musicians" , "Two violins, viola and a cello string group" , "Stringed group" , "Four musicians playing together" , "Group such as the Beatles" .)