Doddle (6)
(Other definitions for picnic that I've seen before include "Outdoor snack" , "something easy to do" , "food for consumption outside" , "Informal meal taken in the open air" , "Outdoor meal (that's easy?)" .)
'doddle' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for breeze that I've seen before include "Something very easy to do (colloq.)" , "Simple task" , "Air current" , "breath of fresh air" , "Soft air" .)
(Other definitions for piece of cake that I've seen before include "It's easy (to eat)" , "Part of Madeira perhaps" , "one can sail through?" , "Easy task" , "slice?" .)