Sherlock Holmes' hat? (11)
I believe the answer is:
deerstalker
'sherlock holmes' hat?' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for deerstalker that I've seen before include "Type of cloth cap" , "Hunter's hat" , "Soft cloth hat" , "Holmes's hat" , "Sherlock Holmes' hat" .)