Group of horse trainers (3)
I believe the answer is:
set
'group' is the definition.
(both can mean a collection)
'of horse trainers' is the wordplay.
'of' indicates a hidden word.
'SET' is hidden in the letters of 'horse trainers'.
(Other definitions for set that I've seen before include "Become solid like jelly" , "become firm" , "Put, coagulate" , "Put down the group of games" , "Become fixed or solid" .)