Stop diets spreading around school (6)
I believe the answer is:
desist
'stop' is the definition.
(desist means to stop)
'diets spreading around school' is the wordplay.
'spreading' indicates an anagram.
'around' is an insertion indicator.
'school' becomes 'S'.
'diets' with letters rearranged gives 'deist'.
'deist' enclosing 's' is 'DESIST'.
(Other definitions for desist that I've seen before include "Stop, abstain" , "'Give over, refrain (6)'" , "Stop, discontinue" , "Abstain, refrain" , "Eject; annoyed" .)