Mad teachers get extremely testy (5)
I believe the answer is:
nutty
'mad' is the definition.
(thesaurus)
'teachers get extremely testy' is the wordplay.
'teachers' becomes 'nut' (National Union of Teachers).
'get' is a charade indicator (letters next to each other).
'extremely' means to remove the middle letters.
'testy' with its centre taken out is 'ty'.
'nut'+'ty'='NUTTY'
(Other definitions for nutty that I've seen before include "Like brazils, say" , "Like, eg, brazils" , "Eccentric - like nougat?" , "Crackers" , "Insane" .)