What reduces Shakespearean old hat (5)
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" .)