Mount rearmost of horses in major road (5)
I believe the answer is:
sinai
'mount' is the definition.
(I've seen this before)
'rearmost of horses in major road' is the wordplay.
'rearmost of horses' becomes 's' (I can't justify this - if you can you should believe this answer much more).
'major road' becomes 'AI' (resembles As, major English road).
's'+'in'+'ai'='SINAI'
Can you help me to learn more?
(Other definitions for sinai that I've seen before include "Mountainous peninsula in Egypt" , "Egyptian desert" , "Law handed down here" , "One showing genetic change" , "Biblical mount" .)