Moderate class, we hear? (6)
I believe the answer is:
lessen
'moderate' is the definition.
(thesaurus)
'class we hear?' is the wordplay.
'class' becomes 'lesson' (I've seen this before).
'we hear?' shows a homophone (sound like).
'lesson' is a homophone of 'LESSEN'.
(Other definitions for lessen that I've seen before include "Contract" , "Cut back or diminish" , "Decrease, diminish" , "Make smaller or fewer" , "decline" .)