Sponge cake serving had you level slab ends up (6)

'sponge' is the definition.
Both the answer and definition are singular nouns.
Maybe you can see a link between them that I can't see?

'cake serving had you level slab ends up' is the wordplay.
'ends' suggests the final letters.
'up' is a reversal indicator (in down clue: letters go upwards).
The final letters of 'cake serving had you level slab' are 'egdulb'.
'egdulb' backwards is 'BLUDGE'.

Can you help me to learn more?