Cake that's special gone off (6)
I believe the answer is:
sponge
'cake' is the definition.
(sponge cake is a kind of cake)
'special gone off' is the wordplay.
'special' becomes 'sp'.
'off' indicates anagramming the letters.
'gone' with letters rearranged gives 'onge'.
'sp'+'onge'='SPONGE'
'that's' acts as a link.
(Other definitions for sponge that I've seen before include "Porous mass for wiping up" , "drunkard or > hanger-on" , "Porous substance found in the ocean" , "mop" , "Sea animal often found in bathroom" .)