Not in theory a laughing matter? (9,4)
I believe the answer is:
practical joke
'not in theory' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
'a laughing matter?' is the wordplay.
I cannot really understand how this works, but
'a' is within the answer.
'matter?' could be 'ti' (Ti is an example) and 'ti' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for practical joke that I've seen before include "Trick played to discomfort someone" , "rag" , "Prank designed to make a fool of its victim" , "Jape - hoax" , "Unkind prank" .)