Tell during class (6)
I believe the answer is:
inform
'tell' is the definition.
(thesaurus)
'during class' is the wordplay.
'during' becomes 'in' (I've seen this in other clues).
'class' becomes 'form' (form class is a kind of class**).
'in'+'form'='INFORM'
(Other definitions for inform that I've seen before include "snitch?" , "Apprise: grass!" , "effective" , "Spill the beans" , "Enlighten" .)