Endlessly bad-tempered note (8)
I believe the answer is:
crotchet
'note' is the definition.
(musical note)
'endlessly bad-tempered' is the wordplay.
I cannot really see how this works, but
'bad' could be 'rot' (I've seen this before) and 'rot' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for crotchet that I've seen before include "In music, it has quarter the time value of a whole note" , "Two quavers as one note" , "Quarter of a semibreve" , "as mark scored" , "perverse notion" .)