dernier ressort

ace in the hole card up one's sleeve last expedient last hope last resource pis aller recourse resort trump card

backup replacement stand-in surrogate

agent alternate assistant auxiliary changeling delegate deputy double dummy equivalent expediency expedient fill in ghost locum makeshift procurator proxy recourse refuge relay relief representative reserve resort resource standby stopgap sub succedaneum successor supplanter supply symbol temp temporary understudy vicar

ghost writer locum tenens pinch-hitter temporary expedient

being entity permanent