Baseball player's casual clothing combination (9)
I believe the answer is:
shortstop
'baseball player's' is the definition.
(shortstop is a kind of baseball player)
'casual clothing combination' is the wordplay.
'casual clothing' becomes 'shorts'.
'combination' becomes 'top' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'shorts'+'top'='SHORTSTOP'
(Other definitions for shortstop that I've seen before include "position in American sport" , "US player" , "Opening move" , "Fielding position (baseball)" , "Baseball player" .)