Long in laughter, he is (5)
I believe the answer is:
hyena
'he is' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
'long in laughter' is the wordplay.
'long' becomes 'yen' (synonyms).
'in' indicates putting letters inside.
'laughter' becomes 'ha' (I can't justify this - if you can you should believe this answer much more).
'yen' put inside 'ha' is 'HYENA'.
Can you help me to learn more?
(Other definitions for hyena that I've seen before include "Carnivorous mammal" , "Carrion-feeding carnivore" , "Scavenging carnivore" , "one may be spotted" , "Dog-like carrion eater" .)