Belonging to a man in the Sandwich Islands (3)
I believe the answer is:
his
'belonging to a man' is the definition.
(I've seen this before)
'in the sandwich islands' is the wordplay.
'in' says the answer is hidden in the clue.
'HIS' can be found hidden inside 'the sandwich islands'.
(Other definitions for his that I've seen before include "Belonging to that man" , "that chap's" , "To - Coy Mistress (Marvell)" , "It belongs to him" , "Belonging to the male" .)