Sit in on, as a class (5)
I believe the answer is:
audit
'sit in on as a class' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for audit that I've seen before include "Roman Aquae Sulis" , "Official inspection of company accounts" , "Review firm's accounts, annual check" , "vetting of books" , "Inspect accounts" .)