Serenity shown by sport star (5)
I believe the answer is:
peace
'serenity' is the definition.
(I know that serenity can be written as peace)
'by sport star' is the wordplay.
'by' means one lot of letters go next to another.
'sport' becomes 'PE' (physical education).
'star' becomes 'ace' (synonyms).
'pe' put next to 'ace' is 'PEACE'.
'shown' acts as a link.
(Other definitions for peace that I've seen before include "Calmness" , "Quiet - lack of warfare" , "A Christmas wish" , "Repose, tranquillity" , "Quiet companion" .)