Headdress seen in Tahiti, a rarity (5)
I believe the answer is:
tiara
'headdress' is the definition.
(I know that tiara is a type of jeweled headdress)
'in tahiti a rarity' is the wordplay.
'in' indicates the answer is hidden within the clue.
'TIARA' can be found hidden inside 'tahiti a rarity'.
'seen' is the link.
(Other definitions for tiara that I've seen before include "Woman's headdress for very formal occasions" , "it goes to one's head!" , "It might go to one's head" , "Jewelled, ornamental headgear" , "Pope's triple-tiered crown" .)