In jest, might be termed a popular rhyme (7)
I believe the answer is:
jocular
'in' is the definition.
Both the answer and definition are adjectives. Perhaps they are linked in a way I don't understand?
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for jocular that I've seen before include "Humorous or playful" , "With jokes and good humour" , "funny" , "Humorous, joking" , "Given to telling gags (and having fun)" .)