Reduces classes for auditors (7)
I believe the answer is:
lessens
'reduces' is the definition.
(to lessen is to reduce or decrease)
'classes for auditors' is the wordplay.
'classes' becomes 'lessons' (school classes).
'for auditors' indicates a 'sounds like' (homophone) clue (auditor can mean someone listening).
'lessons' sounds like 'LESSENS'.
(Other definitions for lessens that I've seen before include "Abates" , "Belittles" , "dies down" , "Disparages" .)