"__!" ... "Polo!" (5)
(Other definitions for shirt that I've seen before include "Garment for upper male body, usually cotton" , "Man's garment" , "Male garment worn with tie" , "One may lose it at the bookie's" , "tailored garment" .)
'polo' is the definition.
(I know that polo given name Marco)
This is the entire clue.
(Other definitions for marco that I've seen before include "gondolier" , "- - Polo, traveller" .)