Success by Mark, an assassin (6)
I believe the answer is:
hitman
'assassin' is the definition.
(I've seen this before)
'success by mark an' is the wordplay.
'success' becomes 'hit' (a hit is a commercial success).
'by' means one lot of letters go next to another.
'mark' becomes 'm'.
'hit'+'m'+'an'='HITMAN'
(Other definitions for hitman that I've seen before include "killer" , "Assassin" , "offer?" , "Hired gun" .)