It's in our blood (5)
I believe the answer is:
serum
'blood' is the definition.
(thesaurus)
I can't explain the rest of the clue.
Can you help me to learn more?
(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" .)