Filly in calm area (4)
I believe the answer is:
mare
'filly' is the definition.
(thesaurus)
'in calm area' is the wordplay.
'in' indicates a hidden word.
'MARE' is hidden within 'calm area'.
(Other definitions for mare that I've seen before include "Female equine animal" , "A night-time strangler" , "For classicists, sea" , "Nag" , "Adult female zebra" .)