Yet it may be quite a pedestrian clause (5)
I believe the answer is:
rider
'clause' is the definition.
(rider is a kind of clause)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for rider that I've seen before include "Motorcyclist" , "One backing a horse" , "Contractual clause" , "One in the saddle" , "in the dressing room before the show?" .)