Teacher made knots in his hair (12)
I believe the answer is:
headmistress
'teacher' is the definition.
(I've seen this before)
'made knots in his hair' is the wordplay.
'knots' is an anagram indicator (I've seen 'knotted' mean this).
'in' means one lot of letters goes inside another.
'hair' becomes 'tress' (tress is a kind of hair).
'made' anagrammed gives 'eadm'.
'eadm' inserted inside 'his' is 'headmis'.
'headmis'+'tress'='HEADMISTRESS'
(Other definitions for headmistress that I've seen before include "School principal" , "Person in charge of a school" , "Female school leader" , "Teacher" .)