Bad-tempered note about not drinking (5)
I believe the answer is:
ratty
'bad-tempered' is the definition.
(I've seen this before)
'note about not drinking' is the wordplay.
'note' becomes 'ray' (I've seen this before).
'about' is an insertion indicator.
'not drinking' becomes 'tt' (short for teetotal).
'ray' going around 'tt' is 'RATTY'.
(Other definitions for ratty that I've seen before include "Irritable - Mole's friend!" , "Fictional mole's chum" , "Short-tempered" , "water vole, literally" , "One messing about in a boat" .)