Check cheats before school (9)
I believe the answer is:
constrain
'check' is the definition.
(constraining is a kind of checking)
'cheats before school' is the wordplay.
'cheats' becomes 'cons' (con is a kind of cheat).
'before' means one lot of letters go next to another.
'school' becomes 'train' (training is a kind of schooling**).
'cons'+'train'='CONSTRAIN'
(Other definitions for constrain that I've seen before include "Hold back - non-racist (anag)" , "Compel; constrict" , "Restrict scope of (an activity)" , "Check" , "Compel - limit" .)