Posted these earlier but think the thread got swamped by others, also Mamya has already answered some in the interim. The rest are....
35a. Ring negotiator up (10) - SHOWJUMPER
I know showjumping can take place in a ring, but so can a million other things (plus they negotiate fences not rings) - UP indicates it's to do do with horses but surely there's more to it - what am I not seeing?
37d. Resist force when beaten, reportedly (4,1,5) - MAKE A STAND (Resist)
I think MAKE = Force, and I presume the rest is a homophone, because of the 'Reportedly', but of what?
48d. Asian equine plague described by poet's ended (6) - ONAGER (Asian equine)