Refrain from eating first bit of undercooked cake (5)
I believe the answer is:
donut
'cake' is the definition.
(donut is a kind of cake)
'refrain from eating first bit of undercooked' is the wordplay.
'refrain from' becomes 'dont' (do not).
'eating' means one lot of letters goes inside another.
'first bit of undercooked' becomes 'u' (dst letter of 'undercooked').
'dont' going around 'u' is 'DONUT'.
(Other definitions for donut that I've seen before include "treat for Homer Simpson" , "cake in the American way" , "ring coming across the Atlantic" , "cake, American-style" , "Sugary deep-fried cake" .)