Simply a piece of cake (6)
I believe the answer is:
facile
'a piece of cake' is the definition.
(I've seen this before)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for facile that I've seen before include "Easy to accomplish" , "Simplistic" , "Smooth" , "A piece of cake" , "Superficial - glib" .)