Broncobuster's horse (3)
I believe the answer is:
cob
'horse' is the definition.
(type of horse)
'broncobuster's' is the wordplay.
'COB' is hidden within the letters.
But, I'm uncertain how this is indicated.
Can you help me to learn more?
(Other definitions for cob that I've seen before include "Source of bread" , "A loaf; a nut; a horse" , "Horse, male swan and loaf of bread" , "Swan of Jacobean origin" , "Hazelnut" .)