Laughing at isn't funny, it's irritating (2,8)
I believe the answer is:
in stitches
'irritating' is the definition.
I know nothing about this answer so I cannot tell whether this works.
'laughing at isn't funny' is the wordplay.
'laughing' becomes 'itches' (I can't explain this - if you can you should give a lot more credence to this answer).
'at' says to put letters next to each other (I've seen this in other clues).
'funny' is an anagram indicator.
'isnt' with letters rearranged gives 'inst'.
'itches' put after 'inst' is 'IN STITCHES'.
'it's' acts as a link.
Can you help me to learn more?
(Other definitions for in stitches that I've seen before include "Helpless with laughter" , "extremely amused" , "Following a visit to the theatre one may be" , "After an operation" , "much affected by comedy?" .)