### Clue: Top classes well behaved over exercise suddenly (8)
### Answer: ABRUPTLY 
 ### Explanation: Definition: suddenly
 A,B(the two top classes;the best and the next best in quality or rank) + RULY(orderly and well-behaved, a back-formation from "unruly") **containing**(over, as in "covering") PT(abbrev. for "physical training";physical exercise) .