Free from grime when brought into class (8)
I believe the answer is:
degrease
'free from grime' is the definition.
'when brought into class' is the wordplay.
'when' becomes 'as'.
'brought into' indicates putting letters inside.
'class' becomes 'degree'.
'as' inserted inside 'degree' is 'DEGREASE'.
(Other definitions for degrease that I've seen before include "Lose fat" , "Remove oily substance from" , "Remove oily matter from" , "Remove fat" .)