The two of us, informally, showing where all other down answers may be found (4)
I believe the answer is:
menu
'where all other down answers may be found' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'the two of us informally' is the wordplay.
I cannot quite understand how this works, but
'us' could be 'me' ('us' can mean 'mean' in some UK dialects) and 'me' is present in the answer.
The remaining letters 'nu' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'showing' is the link.
(Other definitions for menu that I've seen before include "List of dishes available in restaurant" , "Food programme of restaurant" , "List of available food and drink" , "Tariff" , "List of fare" .)