Horse will climb (5)
I believe the answer is:
mount
I believe this clue is a double definition.
'horse' is the first definition.
(I know that mount is a type of horse)
'climb' is the second definition.
(I know that climb can be written as mount)
'will' is the link.
(Other definitions for mount that I've seen before include "The height" , "Get on - horse" , "Fix onto a backing" , "hill" , "Steed" .)