Maybe our teacher's not to be trusted (11)
I believe the answer is:
treacherous
'not to be trusted' is the definition.
(I've seen this before)
'maybe our teacher's' is the wordplay.
'maybe' indicates anagramming the letters.
'our'+'teachers'='ourteachers'
'ourteachers' is an anagram of 'TREACHEROUS'.
(Other definitions for treacherous that I've seen before include "Disloyal and untrustworthy" , "Perfidious" , "Disloyal; dangerous" , "covered in ice?" , "Not true" .)