Trying class turning up before sound of bell (7)
I believe the answer is:
testing
'trying' is the definition.
('test' can be a synonym of 'try')
'class turning up before sound of bell' is the wordplay.
'class' becomes 'set' (I've seen this in other clues).
'turning up' says the letters should be written backwards.
'before' is a charade indicator (letters next to each other).
'sound of bell' becomes 'ting' (I've seen this in other clues).
'set' back-to-front is 'tes'.
'tes'+'ting'='TESTING'
(Other definitions for testing that I've seen before include "tiresome" , "of a difficult nature" , "stressful" , "Examining" , "Subjecting to 15 ac" .)