Trios of trios (6)
I believe the answer is:
nonets
'trios of trios' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for nonets that I've seen before include "Pieces of music for groups of nine" , "Groups of nine musicians" , "Group of players" , "musical pieces" , "Compositions for nine performers" .)