Droll nonsense -- could it bring the house down? (3,3)
I believe the answer is:
dry rot
'could it bring the house down?' is the definition.
(dry rot might cause a house to collapse)
'droll nonsense' is the wordplay.
'droll' becomes 'dry' (I can't justify this - if you can you should believe this answer much more).
'nonsense' becomes 'rot' (rot can mean nonsense or rubbish).
'dry'+'rot'='DRY-ROT'
Can you help me to learn more?
(Other definitions for dry rot that I've seen before include "Wood-destroying fungus" , "Decay of wood poorly ventilated" , "fungal infection" , "it may make one's beam disappear" , "domestic problem" .)