Knight, said to be on horse, is overcome (8)
I believe the answer is:
surmount
'is overcome' is the definition.
(I've seen this before)
'knight said to be on horse' is the wordplay.
'knight' becomes 'sir' (a knight has the title Sir).
'said to be' indicates a 'sounds like' (homophone) clue.
'on' means one lot of letters go next to another.
'horse' becomes 'mount' (mount is a kind of horse).
'sir' sounds like 'sur'.
'sur'+'mount'='SURMOUNT'
(Other definitions for surmount that I've seen before include "Prevail over" , "Get on top of" , "Stand on top of" , "Tight embrace" , "Conquer, get the better of" .)