Mega-Tory -- he worked out principles designed to bring success (4,6)
I believe the answer is:
game theory
'principles designed to bring success' is the definition.
'mega-tory he worked out' is the wordplay.
'worked out' is an anagram indicator.
'mega'+'tory'+'he'='megatoryhe'
'megatoryhe' is an anagram of 'GAME THEORY'.
(Other definitions for game theory that I've seen before include "this might help with decision making" , "part of mathematics" .)