Sit-in possibly cracked dome (4)
I believe the answer is:
demo
'sit-in' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'cracked dome' is the wordplay.
'cracked' indicates anagramming the letters.
'dome' is an anagram of 'DEMO'.
'possibly' is the link.
Can you help me to learn more?
(Other definitions for demo that I've seen before include "presentation" , "exhibit" , "In brief, a public event as of protest under dome" , "illustration" , "Brass instrument valve" .)