Headdress so long one wears (5)
I believe the answer is:
tiara
'headdress' is the definition.
(tiara is a kind of headdress)
'so long one wears' is the wordplay.
'so long' becomes 'ta-ra' (informal way of saying goodbye).
'one' becomes 'i' (Roman numeral).
'wears' indicates putting letters inside.
'tara' enclosing 'i' is 'TIARA'.
(Other definitions for tiara that I've seen before include "Ornamental crescent-shaped headdress or Pope's diadem" , "Half a crown!" , "Half-crown?" , "pope may have been topped by one" , "Jewelled head band" .)