Deserter's charge reduced (3)
I believe the answer is:
rat
'deserter's' is the definition.
(both can mean a traitor)
'charge reduced' is the wordplay.
'charge' becomes 'rate' (rate is a kind of charge).
'reduced' means to remove the last letter.
'rate' with its final letter removed is 'RAT'.
(Other definitions for rat that I've seen before include "Change sides" , "one not going down with ship" , "Cad" , "Sewer rodent" , "shopper" .)