Change umpire or melees start (6)
I believe the answer is:
reform
'change' is the definition.
(reform is a kind of change)
'umpire or melees start' is the wordplay.
'umpire' becomes 'ref' (short for referee).
'start' indicates taking the first letters.
The initial letter of 'melees' is 'm'.
'ref'+'or'+'m'='REFORM'
(Other definitions for reform that I've seen before include "Go straight" , "Correct(ion of) abuses" , "Change one's ways (for the better?)" , "Mend one's ways" , "Removal of abuses" .)