Crossword Genius

It can ruin a good deal (3,3)

Ross

I believe the answer is:

dry rot

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

'it' is the definition.
The definition suggests a singular noun which matches the answer.

'ruin a good deal' is the wordplay.
I cannot really see how this works, but
'ruin' could be 'rot' and 'rot' is found in the answer.
The remaining letters 'dry' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...

'can' acts as a link.

Can you help me to learn more?

(Other definitions for dry rot that I've seen before include "Decay of wood poorly ventilated" , "Decay of timber caused by fungus" , "Disease of wood not well ventilated" , "hidden decay" , "Decay of badly ventilated wood" .)

I've seen this clue in The Globe and Mail.
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