Dominant reason for a horse being tired? (10)
I believe the answer is:
overriding
'dominant' is the definition.
(similar in meaning)
'reason for a horse being tired?' is the wordplay.
I cannot really see how this works, but
'horse' could be 'riding' (riding horse is a kind of horse) and 'riding' is found in the answer.
The remaining letters 'over' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for overriding that I've seen before include "most important" , "taking precedence" .)