Group of fish disturb stream of a teacher (12)
I believe the answer is:
schoolmaster
'teacher' is the definition.
(I know that schoolmaster is a type of teacher)
'group of fish disturb stream' is the wordplay.
'group of fish' becomes 'school' (I've seen this in another clue).
'disturb' indicates an anagram (I've seen 'disturbed' mean this (the order of letters is disturbed)).
'stream' is an anagram of 'master'.
'school'+'master'='SCHOOLMASTER'
'of a' acts as a link.
(Other definitions for schoolmaster that I've seen before include "Male teacher" , "The classroom (anag)" , "One teaching" .)