“I say there!”, mostly about laughter (8)
I believe the answer is:
hysteria
'laughter' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'i say there mostly about' is the wordplay.
'mostly' means to remove the last letter (most but not all of the word).
'about' indicates anagramming the letters.
'i'+'say'+'there'='isaythere'
'isaythere' with its final letter taken off is 'isayther'.
'isayther' anagrammed gives 'HYSTERIA'.
Can you help me to learn more?
(Other definitions for hysteria that I've seen before include "display of wild emotion" , "Uncontrolled emotion" , "Ogden -, US poet" , "State of violent mental agitation" , "highly emotional state" .)