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 in amongst the letters.
But, I am not sure 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" .)