Crossword Genius

A good 'un, but showing signs of breaking (7)

Ross

I believe the answer is:

cracker

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

'a good' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.

''un but showing signs of breaking' is the wordplay.
I cannot really understand how this works, but
'breaking' could be 'crack' (crack is a kind of breaking) and 'crack' is present in the answer.
The remaining letters 'er' 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 cracker that I've seen before include "pulled at Christmas?" , "Dry biscuit; Christmas item" , "Xmas table extra" , "Biscuit for cheese" , "Savoury biscuit" .)

I've seen this clue in the Evening Standard.
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