Empty tune by kind of jazz quartet (6)
I believe the answer is:
tetrad
'quartet' is the definition.
('tetrad' can be a synonym of 'quartet')
'empty tune by kind of jazz' is the wordplay.
'empty' means to remove the middle letters.
'by' means one lot of letters go next to another.
'kind of jazz' becomes 'trad'.
'tune' with its middle taken out is 'te'.
'te'+'trad'='TETRAD'
(Other definitions for tetrad that I've seen before include "quartet" , "number of Gospel writers" , "Group or set of four" .)