Senior teacher takes top stream after reorganisation (10)
I believe the answer is:
headmaster
'senior teacher' is the definition.
(teacher in charge of a school)
'top stream after reorganisation' is the wordplay.
'top' becomes 'head' (head is a kind of top).
'after reorganisation' indicates an anagram.
'stream' anagrammed gives 'master'.
'head'+'master'='HEADMASTER'
'takes' is the link.
(Other definitions for headmaster that I've seen before include "School manager" , "Top (male) teacher" , "Principal" .)