Right to arrest a deserter (3)
I believe the answer is:
rat
'deserter' is the definition.
(thesaurus)
'right to arrest a' is the wordplay.
'right' becomes 'rt'.
'to arrest' indicates putting letters inside.
'rt' enclosing 'a' is 'RAT'.
(Other definitions for rat that I've seen before include "Despicable person" , "one who betrayed" , "Rodent; informer (colloq.)" , "that gets out of stricken vessel" , "one escaping doomed vessel?" .)