Knight, say, following success as slayer (6)
I believe the answer is:
hitman
'slayer' is the definition.
(I know that hitman is a type of slayer)
'knight say following success' is the wordplay.
'knight say' becomes 'man' (knight is a kind of man).
'following' says to put letters next to each other.
'success' becomes 'hit' (hit is a kind of success).
'man' put after 'hit' is 'HITMAN'.
'as' is the link.
(Other definitions for hitman that I've seen before include "Assassin" , "killer" , "offer?" , "Hired gun" .)