Byzantine has big belt, plumed hat? (5)
I believe the answer is:
shako
'plumed hat?' is the definition.
'shako' can be an answer for 'hat?' (shako is a kind of hat). I'm unsure of the 'plumed' bit.
'byzantine has big belt' is the wordplay.
'byzantine' indicates anagramming the letters (byzantine means complex or confusing).
'big belt' becomes 'ko' (I can't explain this - if you can you should give a lot more credence to this answer).
'has' with letters rearranged gives 'sha'.
'sha'+'ko'='SHAKO'
Can you help me to learn more?
(Other definitions for shako that I've seen before include "Old headgear" , "old military item" , "Ceremonial hat" , "Capital cover" , "One covering for military head" .)