Foe somewhat serene, mysteriously (5)
I believe the answer is:
enemy
'foe' is the definition.
(I know that foe can be written as enemy)
'somewhat serene mysteriously' is the wordplay.
'somewhat' says the answer is hidden in the clue.
'ENEMY' is hidden in the letters of 'serene mysteriously'.
(Other definitions for enemy that I've seen before include "Foe, adversary" , "Division" , "Hostile opponent" , "one in opposition" , "other side (like 16)" .)