deputize

To officially empower, authorize, pronounce, and establish (someone) to be a deputy.

to officially empower and authorize (someone) to act as a substitute in one's role or office

I deputize you to act for me while I'm away.

To act as a substitute for a person in their role or office

Gringoire, on the other hand, stuck with equal firmness to the fact that he was deputizing for a brother officer who was sick — so sick that he had mislaid his orders.

What he wanted was someone who could really deputize for him when he was away on business – clearly not the job for a secretary but, because of years of experience and a willingness to take responsibility, one that the previous holder of the post had taken on.