Clique of horse trainers (3)
I believe the answer is:
set
'clique' is the definition.
(I've seen this before)
'of horse trainers' is the wordplay.
'of' indicates a hidden word.
'SET' can be found hidden inside 'horse trainers'.
(Other definitions for set that I've seen before include "box?" , "exclusive party" , "Put, coagulate" , "Mount (gem); scenery" , "Major division of tennis match" .)