It can ruin a good deal (3,3)
I believe the answer is:
dry rot
'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" .)