Blimey guvnor - sounds like such a fast horse! (7)
I believe the answer is:
courser
'horse' is the definition.
(a very fast horse)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for courser that I've seen before include "Runner" , "Swift horse" , "Hunter of hares - speedy horse or bird" , "One hunting with dogs" .)