In default of something
Definition 1 of 2
Pro Tip 1/3Expectation Absence
Use when expected action or thing doesn't happen or exist, indicating a fallback situation. 
In default of expert advice, we relied on our intuition.
Pro Tip 2/3Implied Necessity
Suggests that there's a necessary condition missing, often requiring an alternative solution. 
In default of sufficient funds, the trip was canceled.
Pro Tip 3/3Plan B Scenario
Implies a secondary plan or action is in place when the primary one fails or doesn't occur. 
In default of the main speaker, John will give the lecture.