Greediest man's silly argument (12)
I believe the answer is:
disagreement
'argument' is the definition.
(I've seen this before)
'greediest man's silly' is the wordplay.
'silly' indicates an anagram.
'greediest'+'man'='greediestman'
'greediestman' anagrammed gives 'DISAGREEMENT'.
(Other definitions for disagreement that I've seen before include "Difference of opinion" , "dispute" , "Argument" , "Conflict" , "Quarrel" .)