Old king retired, carrying current coat (5)
I believe the answer is:
tunic
'coat' is the definition.
(I have seen 'Belted coat ' mean 'tunic' so perhaps 'coat' could also mean 'tunic')
'old king retired carrying current' is the wordplay.
'old king' becomes 'cnut' (King Cnut).
'retired' is a reversal indicator.
'carrying' indicates putting letters inside.
'current' becomes 'i' (symbol for electric current).
'cnut' written backwards gives 'tunc'.
'tunc' enclosing 'i' is 'TUNIC'.
(Other definitions for tunic that I've seen before include "Loose sleeveless garment" , "School garment" , "Loose fitting light cloak extending to the hip or lower" , "Shift" , "Clobber" .)