Hold the fort
Definition 1 of 1
Pro Tip 1/3Temporary Responsibility
Indicates that someone is temporarily taking on another's duties. 
While the manager is on vacation, I will hold the fort.
Pro Tip 2/3Underlying Trust
Implies a level of trust placed in the person asked to take charge. 
You're reliable, so I'm sure you can hold the fort while I'm away.
Pro Tip 3/3Not Ownership
Suggests looking after something, but not owning or having full permanent control over it. 
I'm just holding the fort until the new chef arrives.