Travel across top of gentle peak (5)
I believe the answer is:
ridge
'peak' is the definition.
(I've seen this before)
'travel across top of gentle' is the wordplay.
'travel' becomes 'ride' (ride is a kind of travel**).
'across' indicates putting letters inside.
'top of' says to take the initial letters.
The initial letter of 'gentle' is 'g'.
'ride' enclosing 'g' is 'RIDGE'.
(Other definitions for ridge that I've seen before include "Hog's-back" , "Line of raised ground" , "Long raised band" , "watershed ..." , "Narrow hill-range" .)