Stop uprising, generally when classes leave (8)

'classes leave' is the definition.
The definition suggests a singular noun which matches the answer.

'stop uprising generally' is the wordplay.
'stop' becomes 'avert' (to avert something is to stop or prevent it).
'uprising' says the letters should be written backwards.
'generally' becomes 'lly' (I can't explain this - if you can you should give a lot more credence to this answer).
'avert' reversed gives 'treva'.
'treva'+'lly'='TREVALLY'

'when' is the link.

Can you help me to learn more?