And to assemble in the dining-room would be very silly (7)
I believe the answer is:
madness
'very silly' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'and to assemble in the dining-room' is the wordplay.
'to assemble' is an anagram indicator.
'in the' means one lot of letters goes inside another.
'dining room' becomes 'mess' (mess is a kind of dining room).
'and' anagrammed gives 'adn'.
'adn' going inside 'mess' is 'MADNESS'.
'would be' is the link.
Can you help me to learn more?
(Other definitions for madness that I've seen before include "Group" , "Absurdity" , "Irrationality" , "Utter foolishness" , "Lunacy - ska/pop band" .)