Die of seizure after joke’s backfired (5,2)
I believe the answer is:
snuff it
'die' is the definition.
(snuffing it is a kind of diing)
'seizure after joke's backfired' is the wordplay.
'seizure' becomes 'fit' (fit is a kind of seizure).
'after' is a charade indicator (letters next to each other).
'joke's' becomes 'funs' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'backfired' is a reversal indicator.
'funs' written backwards gives 'snuf'.
'fit' put after 'snuf' is 'SNUFF IT'.
'of' acts as a link.
Can you help me to learn more?
(Other definitions for snuff it that I've seen before include "Die (slang)" , "Pop one's clogs!" , "Kick the bucket" , "Peg out" , "Die (having put the candle out?)" .)