Player’s derby (6)
I believe the answer is:
bowler
I believe this clue is a double definition.
'player's' is the first definition.
(bowler is a kind of player)
'derby' is the second definition.
(synonyms)
(Other definitions for bowler that I've seen before include "something more substantial than a cap" , "Cricketer and round hat" , "Hard hat for cricketer?" , "Type of hat" , "Round black hat with brim" .)