Piece of cake to eat out (6)
I believe the answer is:
picnic
I believe this is a double definition.
'piece of cake' is the first definition.
(thesaurus)
'eat out' is the second definition.
(I've seen this before)
'to' acts as a link.
(Other definitions for picnic that I've seen before include "Informal outdoor summer meal" , "Eat out" , "is difficult" , "Informal meal taken in the open air" , "Piece of cake!" .)