Crossword Genius

An inedible cake? (4)

Ross

I believe the answer is:

soap

I'm a little stuck... Click here to teach me more about this clue!

'an inedible cake?' is the definition.
(I've seen this in another clue)

'an inedible' is the wordplay.
I cannot quite see how this works, but
'an' could be 'a' and 'a' is present in the answer.
The remaining letters 'sop' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for soap that I've seen before include ". . . . opera, sentimentalised radio or tv series" , "Bathroom item" , "Cleansing bar" , "It has detergent properties" , "bar in Bath?" .)

I've seen this clue in the Evening Standard.
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