Unlimited thread for wrapping round my hat (7)
I believe the answer is:
tricorn
'hat' is the definition.
(tricorn is a kind of hat)
'unlimited thread for wrapping round my' is the wordplay.
'unlimited thread for' becomes 'trin' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'wrapping round' means one lot of letters goes inside another.
'my' becomes 'cor' (both are expressions of surprise).
'trin' enclosing 'cor' is 'TRICORN'.
Can you help me to learn more?
(Other definitions for tricorn that I've seen before include "old hat" , "head wear" , "Cocked hat with brim turned up on three sides" , "Hat with upturned brim" , "headgear" .)