Eat out -- it's a piece of cake (6)
I believe the answer is:
picnic
I believe this is a double definition.
'eat out' is the first definition.
(I've seen this before)
'a piece of cake' is the second definition.
(thesaurus)
'it's' is the link.
(Other definitions for picnic that I've seen before include "Meal outdoors" , "Eat out" , "external courses?" , "Take a meal outside" , "Outdoor repast" .)