For which low gear is highly reasonable (5)
I believe the answer is:
hills
'for which low gear is highly reasonable' is the definition.
The definition suggests a singular noun which matches the answer.
This is all the clue.
Can you help me to learn more?
(Other definitions for hills that I've seen before include "See 7" , "Heights; tors" , "Elevations of land" , "High terrain" , "Mounts" .)