Serenity, in classical mythology (4)
I believe the answer is:
calm
'serenity' is the definition.
(I've seen this in another clue)
'in classical mythology' is the wordplay.
'in' indicates the answer is hidden within the clue.
'CALM' is hidden within 'classical mythology'.
(Other definitions for calm that I've seen before include "Still - force 0 on the Beaufort scale" , "Peaceful and unexcited" , "Unruffled, not excited" , "Cool, composed" , "Placid, unexcited" .)