Bad-tempered blackleg, extremely testy (5)
I believe the answer is:
ratty
'bad-tempered' is the definition.
(I've seen this before)
'blackleg extremely testy' is the wordplay.
'blackleg' becomes 'rat' (synonyms).
'extremely' means to remove the middle letters.
'testy' with its centre taken out is 'ty'.
'rat'+'ty'='RATTY'
(Other definitions for ratty that I've seen before include "with pests" , "Fictional mole's chum" , "Moley's friend" , "One messing about in a boat" , "Cross - character in "The Wind in the Willows"" .)