Cheat upset umpire holding machine gun (5)
I believe the answer is:
faker
'cheat' is the definition.
(I know that faker is a type of cheat)
'upset umpire holding machine gun' is the wordplay.
'upset' is a reversal indicator.
'umpire' becomes 'ref' (short for referee).
'holding' is an insertion indicator.
'machine gun' becomes 'ak' (short for ak-47).
'ref' backwards is 'fer'.
'fer' enclosing 'ak' is 'FAKER'.
(Other definitions for faker that I've seen before include "counterfeiter" , "Fraud" .)