Soft piece of cake (4)
I believe the answer is:
easy
I believe this is a double definition.
'soft' is the first definition.
(I know that soft can be written as easy)
'piece of cake' is the second definition.
(I've seen this before)
(Other definitions for easy that I've seen before include "Posing no difficulty" , "without difficulty" , "Achieved without effort" , "Simple or uncomplicated" , "... Rider (film)" .)