Master almost certain to get horse (8)
I believe the answer is:
'master' is the definition.
(I know that master can be written as surmount)
'almost certain to get horse' is the wordplay.
'almost' means to remove the last letter (almost the whole word is kept).
'certain' becomes 'sure' (synonyms).
'to get' is a charade indicator (letters next to each other).
'horse' becomes 'mount' (mount is a kind of horse).
'sure' with its final letter taken away is 'sur'.
(Other definitions for surmount that I've seen before include "Overcome a difficulty, get over" , "Prevail over" , "Conquer, get the better of" , "Get on top of" , "Overcome (obstacle)" .)