Group of four (6)
'group of four' 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 performers" , "Two violins, viola and a cello string group" , "Set of four people" , "Stringed group" , "like a string one?" .)
'group of four' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tetrad that I've seen before include "Group or set of four" , "number of Gospel writers" , "quartet" .)