Piece of cake and vegetables ending in tummy — a picnic! (4-5)
I believe the answer is:
easy-peasy
'a picnic' is the definition.
(both can mean easy or simple)
'piece of cake and vegetables ending in tummy' is the wordplay.
'piece of cake' becomes 'easy' (I've seen this in another clue).
'and' says to put letters next to each other.
'vegetables' becomes 'peas' (peas are a type of vegetable).
'ending in' indicates one should take the final letters.
The last letter of 'tummy' is 'y'.
'easy'+'peas'+'y'='EASY-PEASY'
(Other definitions for easy-peasy that I've seen before include "walk in the park" , "Very simple (colloq.)" , "A doddle" , "Like falling off a log (informal)" , "Very simple to achieve" .)