Word that describes a stand in or a frontman? [duplicate]

You might be thinking of proxy

Mw.com

3 : a person authorized to act for another : procurator

Sample sentences

Words in a Sentence

When my husband and I are out of the country, my sister is the proxy who signs legal documents for our children.

John's proxy has been authorized to vote for him during his absence.

During the action sequence, a stand-in will act as a proxy for the A-list actor.

Commonly used in the contexts of proxy war and Munchausen's syndrome by proxy.

Wikipedia

A proxy war is an armed conflict between two states or non-state actors which act on the instigation or on behalf of other parties that are not directly involved in the hostilities.

Wikipedia

Munchausen syndrome by proxy (MSbP), is a condition in which a caregiver creates the appearance of health problems in another person, typically their child.