Flu fighter (5)

I think it could be either:

Which length description are you looking for?:

serum

'flu fighter' is the definition.
(I've seen this in another clue)

This is the entire clue.

(Other definitions for serum that I've seen before include "Inoculation liquid" , "Antidote" , "Watery liquid as from blister, or fluid in blood" , "Constituent of blood" , "Blood component containing antibodies, used in immunisation" .)

I've seen this clue in the LA Times.

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