Group of racehorse trainers (3)
I believe the answer is:
set
'group' is the definition.
(both can mean a collection)
'of racehorse trainers' is the wordplay.
'of' indicates the answer is hidden within the clue.
'SET' can be found hidden inside 'racehorse trainers'.
(Other definitions for set that I've seen before include "Customary < scenery < a TV < series" , "Hard; place" , "Scenery for play or film" , "exclusive party" , "Prescribed" .)