Portuguese coins featured in Treasure Island (4)
I believe the answer is:
reis
'portuguese coins' is the definition.
(plural of 'real', historical currency of Portugual)
'featured in treasure island' is the wordplay.
'featured in' indicates the answer is hidden within the clue.
'REIS' is hidden within 'treasure island'.
(Other definitions for reis that I've seen before include "old money in Lisbon" , "Small branch" .)