Headdress so long that Hoskins must put on (5)
I believe the answer is:
tiara
'headdress' is the definition.
(tiara is a kind of headdress)
'so long that hoskins must put on' is the wordplay.
I cannot quite see how this works, but
'hoskins' could be 'i' (where the crossword setter is 'Hoskins') and 'i' is located in the answer.
'so long' could be 'tara' (informal way of saying goodbye) and 'tara' is located in the leftover letters.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for tiara that I've seen before include "Woman's headdress for very formal occasions" , "Jewelled crescent-shaped headdress" , "Jewelled headband" , "Half a crown!" , "Small crown" .)