Subtly comic nonsense? It could bring down the house (3,3)
I believe the answer is:
dry rot
'it could bring down the house' is the definition.
The definition suggests a singular noun which matches the answer.
'subtly comic nonsense?' is the wordplay.
'subtly comic' becomes 'dry' (I can't justify this - if you can you should give a lot more credence to this answer).
'nonsense?' becomes 'rot' (I've seen this before).
'dry'+'rot'='DRY-ROT'
Can you help me to learn more?
(Other definitions for dry rot that I've seen before include "infestation" , "Decay of wood poorly ventilated" , "Timber disease" , "it may make one's beam disappear" , "Fungus affecting wood" .)