Note returned in time, last class sets off (9)
I believe the answer is:
detonates
'sets off' is the definition.
('detonate' can be a synonym of 'set off')
'note returned in time last class' is the wordplay.
'returned' is a reversal indicator.
'in' is an insertion indicator.
'time' becomes 'date' (a date is a period of time).
'last' suggests the final letters.
The final letter of 'class' is 's'.
'note' reversed gives 'eton'.
'eton' placed within 'date' is 'detonate'.
'detonate'+'s'='DETONATES'
(Other definitions for detonates that I've seen before include "Blows up" , "Sets off" .)