Its stupidity to refuse to accept theres a sit-in (7)
I believe the answer is:
density
'a sit-in' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
'its stupidity to refuse to accept theres' is the wordplay.
I cannot really see how this works, but
an anagram of 'its' is 'sit' which is located in the answer.
'refuse' could be 'deny' (synonyms) and 'deny' is located in the leftover letters.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for density that I've seen before include "Bulk" , "Concentrated condition as of fog" , "Closeness" , "Concentration, compactness" , "Mass/unit vol. (phys.)" .)