Exercises calm to save time? It's morally harmful (10)
I believe the answer is:
pestilence
'it's morally harmful' is the definition.
(pestilence can figuratively mean a moral problem)
'exercises calm to save time?' is the wordplay.
'exercises' becomes 'pe' (Physical Education class).
'calm' becomes 'silence' (I've seen this in another clue).
'to save' means one lot of letters goes inside another.
'time?' becomes 't' (abbreviation).
'silence' going around 't' is 'stilence'.
'pe'+'stilence'='PESTILENCE'
(Other definitions for pestilence that I've seen before include "Fatal epidemic disease" , "that'll plague you" , "Epidemic disease with a high death rate" , "evil force" , "Deadly epidemic disease" .)