This as basis for the annual blow-out? (8,4)
I believe the answer is:
Here is my best explanation:
'this as basis for the annual blow-out?' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
This is the entire clue.
(Other definitions for birthday cake that I've seen before include "Celebratory confection", "Anniversary fare" and "Yearly confection".)