Unintelligible irate lunatic going crazy (12)
I believe the answer is:
inarticulate
'unintelligible' is the definition.
(I've seen this before)
'irate lunatic going crazy' is the wordplay.
'going crazy' indicates an anagram.
'irate'+'lunatic'='iratelunatic'
'iratelunatic' anagrammed gives 'INARTICULATE'.
(Other definitions for inarticulate that I've seen before include "incoherent" , "Not clearly expressed" , "hard to understand" , "Incomprehensible" , "incapable of clear speech" .)