Aristotle misused a calculator! (9)
I believe the answer is:
totaliser
'a calculator' is the definition.
(totaliser is a kind of calculator)
'aristotle misused' is the wordplay.
'misused' indicates anagramming the letters.
'aristotle' with letters rearranged gives 'TOTALISER'.
(Other definitions for totaliser that I've seen before include "Betting system" , "System of speculation" , "Summer" , "system for the better" , "you can bet on it!" .)