Piece of cake -- it's eaten outside (6)
I believe the answer is:
Here is my best explanation:
This clue is a double definition.
'piece of cake' is the first definition.
(I've seen this before)
'it's eaten outside' is the second definition.
(Other definitions for picnic that I've seen before include "Eat out", "Informal excursion with meal in open air", "Meal outdoors", "external courses?", "Piece of cake!", "Informal outdoor summer meal", "outing", "Informal meal in open air for pleasure", "Money for jam" and "Easy task - outdoor event".)