Unusual stir about hackneyed items, things in the 'old hat' category? (8)

'things in the old hat category?' is the definition.
The answer and definition can be both man-made objects as well as being plural nouns.
Maybe you can see an association between them that I can't see?

'unusual stir about hackneyed items' is the wordplay.
'unusual' indicates an anagram.
'about' indicates putting letters inside.
'hackneyed items' becomes 'corn' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'stir' with letters rearranged gives 'tris'.
'tris' enclosing 'corn' is 'TRICORNS'.

Can you help me to learn more?