Travel on horseback across grand elevation (5)
I believe the answer is:
ridge
'elevation' is the definition.
(I know that ridge is a type of elevation)
'travel on horseback across grand' is the wordplay.
'travel on horseback' becomes 'ride' (I've seen this in another clue).
'across' is an insertion indicator.
'grand' becomes 'g' (G is short for grand as in a thousand).
'ride' enclosing 'g' is 'RIDGE'.
(Other definitions for ridge that I've seen before include "Narrow hill-range" , "Long raised band" , "there's a lot of pressure in it" , "Raised strip between two furrows" , "Long, narrow range of hills" .)