There's nothing spongy about it! (4,4)
I believe the answer is:
rock cake
'it' is the definition.
The definition suggests a singular noun which matches the answer.
'there's nothing spongy about' is the wordplay.
I cannot really see how this works, but
'nothing' could be 'o' (O resembles 0) and 'o' is present in the answer.
'about' could be 'ca' (circa) and 'ca' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for rock cake that I've seen before include "Baked tooth-breaker?" , "Rough-looking hard bun" .)