Stop side replacing good man (6)
I believe the answer is:
desist
'stop' is the definition.
(desist means to stop)
'side replacing good man' is the wordplay.
'replacing' indicates an anagram (placing again).
'good man' becomes 'st' (abbreviation for saint).
'side' anagrammed gives 'desi'.
'desi'+'st'='DESIST'
(Other definitions for desist that I've seen before include "Refrain (from doing something)" , "Cease (from doing something)" , "'Give over, refrain (6)'" , "Stop, abstain" , "'Stop, give over (6)'" .)