Found in Tahiti, a rare jewelled headdress (5)
I believe the answer is:
tiara
'jewelled headdress' is the definition.
(I know that tiara is a type of jewelled headdress)
'found in tahiti a rare' is the wordplay.
'found in' indicates a hidden word.
'TIARA' is hidden in the letters of 'tahiti a rare'.
(Other definitions for tiara that I've seen before include "Jewelled head wear" , "Jewelled, ornamental headgear" , "Ornamental crescent-shaped headdress or Pope's diadem" , "Decorative headgear" , "Woman's headdress for very formal occasions" .)