American sports player is best to put on sports clothing (9)
I believe the answer is:
shortstop
'american sports player' is the definition.
(type of baseball player)
'best to put on sports clothing' is the wordplay.
'best' becomes 'top' (top can mean the very best).
'to put on' means one lot of letters go next to another.
'sports clothing' becomes 'shorts' (shorts might be worn when playing sport).
'top' put after 'shorts' is 'SHORTSTOP'.
'is' is the link.
(Other definitions for shortstop that I've seen before include "One in field" , "Opening move" , "Baseball player" , "position in American sport" , "US player" .)