Raising some broncos - outrivalled by expert! (8)
I believe the answer is:
virtuoso
'expert' is the definition.
(virtuoso is a kind of expert)
'raising some broncos outrivalled' is the wordplay.
'raising' is a reversal indicator.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'osoutriv' is hidden within 'broncos outrivalled'.
'osoutriv' back-to-front is 'VIRTUOSO'.
'by' acts as a link.
(Other definitions for virtuoso that I've seen before include "Paganini" , "Showing dazzling skill" , "Person who excels in techniques of singing or playing music" , "Brilliant player" , "Genius" .)