Crossword Genius

What reduces Shakespearean old hat (5)

Ross

I believe the answer is:

shako

'hat' is the definition.
(I know that shako is a type of hat)

I cannot understand how the remainder of the clue works.

(Other definitions for shako that I've seen before include "Tall plumed hat (mil.)" , "soldier's headgear" , "One covering for military head" , "Old headgear" , "Capital cover" .)

I've seen this clue in The Times.
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