The classroom is untidy - and he's responsible (12)
I believe the answer is:
schoolmaster
'he's responsible' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'the classroom is untidy' is the wordplay.
'is untidy' is an anagram indicator.
'the'+'classroom'='theclassroom'
'theclassroom' is an anagram of 'SCHOOLMASTER'.
'and' is the link.
Can you help me to learn more?
(Other definitions for schoolmaster that I've seen before include "Male teacher" , "One teaching" , "The classroom (anag)" .)