Son is relative (3)

I believe the answer is:

sis

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

'son is' is the wordplay.
'son' becomes 's'.
's'+'is'='SIS'

(Other definitions for sis that I've seen before include "Relative's" , "relative" , "Secret Intelligence Service (abbr.)" , "Little sibling" , "Sibling (informal)" .)

I've seen this clue in The Telegraph.

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