Headdress so long one's wearing (5)
I believe the answer is:
tiara
'headdress' is the definition.
(tiara is a kind of headdress)
'so long one's wearing' is the wordplay.
'so long' becomes 'ta-ra' (both can mean goodbye).
'one' becomes 'i' (Roman numeral).
'wearing' is an insertion indicator.
'tara' enclosing 'i' is 'TIARA'.
(Other definitions for tiara that I've seen before include "Jewelled coronet" , "Pope's triple-tiered crown" , "Jewelled headband" , "Ornamental head gear" , "Woman's head ornament" .)