Get on top of (5)
(Other definitions for mount that I've seen before include "Cook, perhaps" , "Steed" , "Climb, stairs say" , "St. Helens, for example" , "Get upon" .)
'get on top of' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for surmount that I've seen before include "Conquer, get the better of" , "Tight embrace" , "Overcome a difficulty, get over" , "Get on top of" , "Overcome (obstacle)" .)