Relative in Christmas Island (3)

I believe the answer is:

sis

'relative' is the definition.
(short for sister)

'in christmas island' is the wordplay.
'in' indicates the answer is hidden within the clue.
'SIS' can be found hidden inside 'christmas island'.

(Other definitions for sis that I've seen before include "Family member (informal)" , "Secret Intelligence Service (abbr.)" , "spies ..." , "Secret Intelligence Service (abbreviation)" , "Girl" .)

I've seen this clue in The Sun.

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