Vegan, say, oddly rejected piece of cake (4)
I believe the answer is:
easy
'piece of cake' is the definition.
(I've seen this in another clue)
'vegan say oddly rejected' is the wordplay.
'oddly rejected' means one should take alternating letters (letters in odd-numbered positions removed).
'vegan'+'say'='vegansay'
The alternating letters of 'vegansay' are 'EASY'.
(Other definitions for easy that I've seen before include "Comparable to pie?" , "Facile" , "Relaxed" , "Foolproof" , "Presenting few difficulties" .)