One donning Great King's spun garment (5)
I believe the answer is:
tunic
'garment' is the definition.
(historical type of garment)
'one donning great king's spun' is the wordplay.
I cannot quite understand how this works, but
'one' could be 'un' (northern English dialect form of 'one') and 'un' is found within the answer.
The remaining letters 'tic' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for tunic that I've seen before include "Shift" , "Knee-length cloak" , "(Policeman's) jacket" , "Belted coat" , "Clobber" .)