Climb or ascend (5)
I believe the answer is:
mount
I believe this clue is a double definition.
'climb' is the first definition.
(I know that climb can be written as mount)
'ascend' is the second definition.
(mounting is a kind of ascending)
(Other definitions for mount that I've seen before include "seat" , "- - Sinai" , "put on launch" , "The height" , "Climb - eg Everest" .)