To some extent, teacher is, he decided, treasured (9)
I believe the answer is:
cherished
'treasured' is the definition.
('cherish' can be a synonym of 'treasure')
'to some extent teacher is he decided' is the wordplay.
'to some extent' indicates the answer is hidden within the clue.
'CHERISHED' is hidden in the letters of 'teacher is he decided'.
(Other definitions for cherished that I've seen before include "Lovingly cared for" , "Greatly valued" , "Precious" , "Held dear" , "clung to" .)