Several horses, a sort of quartet? (6)
I believe the answer is:
string
This clue is a double definition.
'several horses' is the first definition.
(string can mean racehorses sharing a stable)
'a sort of quartet?' is the second definition.
(string quartet)
(Other definitions for string that I've seen before include "Thin piece of cord" , "Number of racehorses" , "Something for tying up" , "Hard to say how long a piece of it is" , "Collective noun for racehorses trained together" .)