Make a mistake and trip (6)
I believe the answer is:
Here is my best explanation:
'trip' is the definition.
(errand is a kind of trip)
'make a mistake and' is the subsidiary indication.
'make a mistake' becomes 'err' ('err' can be a synonym of 'make a mistake').
(Other definitions for errand that I've seen before include "Short trip to perform required task", "Commission, task", "Short trip with a mission", "An ...... of mercy is a journey to relieve distress", "Short journey to get or bring", "Task, message", "Delivery/collection trip", "Commission, message", "Journey to deliver message etc" and "Mission (often run)".)