This hat is out of date (3)
I believe the answer is:
old
This clue could be a double definition.
'this hat' is the first definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'out of date' is the second definition.
(I've seen this in another clue)
'is' acts as a link.
Can you help me to learn more?
(Other definitions for old that I've seen before include "Obsolete" , "Much-used" , "Getting on" , "Venerable" , "At an early stage - at a late stage" .)