Is it possible to delete cache in Mongo?
As per MongoDB documentation here PlanCache.clear() removes all cached query plans for a collection.

The method is only available from the plan cache object of a specific collection; i.e.

db.collection.getPlanCache().clear()


Required Access

On systems running with authorization, a user must have access that includes the planCacheWrite action.

For your further refernce Manage Users and Roles in MongoDB and Collection-Level Access Control in MongoDB