It's in our blood (5)

I believe the answer is:

serum

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'blood' is the definition.
(thesaurus)

I don't understand how the rest of the clue works.

(Other definitions for serum that I've seen before include "Part of blood; vaccine" , "potential shot in the arm?" , "Antidote" , "Constituent of blood" , "Antitoxin derived from blood" .)

I've seen this clue in The Globe and Mail and the Evening Standard.

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