A good 'un, but showing signs of breaking (7)
I believe the answer is:
cracker
'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" .)