Ugh! It's not even your cake! (4)
I believe the answer is:
yuck
'ugh' is the definition.
(I've seen this before)
'not even your cake' is the wordplay.
'not even' indicates alternate letters (remove even-numbered letters).
'your'+'cake'='yourcake'
The alternating letters of 'yourcake' are 'YUCK'.
'it's' is the link.
(Other definitions for yuck that I've seen before include "Expression of disgust; rhymes with male deer" , "Disgusting (interj.)" , "Ugh!" , "Exclamation of disgust" , "That's disgusting!" .)