"So simple!" (4-5)
I believe the answer is:
easy-peasy
'so simple' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for easy-peasy that I've seen before include "Like falling off a log (informal)" , "Very simple to achieve" , "Very simple (colloq.)" , "walk in the park" , "A doddle" .)