Mull over vital fluid changing sides (5)

I believe the answer is:

brood

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'mull over' is the definition.
(brooding is a kind of mulling over)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for brood that I've seen before include "Think resentfully about family of children?" , "Worry over" , "Meditate, fret" , "Ponder mournfully" , "Think resentfully - about young birds?" .)

I've seen this clue in The Hindu.

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