Aristotle tinkered with system for the better (9)
I believe the answer is:
totaliser
'system for the better' is the definition.
(I've seen this before)
'aristotle tinkered with' is the wordplay.
'tinkered with' is an anagram indicator.
'aristotle' is an anagram of 'TOTALISER'.
(Other definitions for totaliser that I've seen before include "Betting system" , "system for the better" , "Summer" , "you can bet on it!" , "Risky system" .)