Stop the flow in the system (4)

I believe the answer is:

stem

I'm a little stuck... Click here to teach me more about this clue!

'stop the flow' is the definition.
(I've seen this before)

'the system' is the wordplay.
'STEM' is hidden within the letters.
But, I'm unsure how the hidden word is indicated.

'in' acts as a link.

Can you help me to learn more?

(Other definitions for stem that I've seen before include "Staunch - support" , "branch of family" , "Ski turn and plant support" , "Type of cell" , "part of a ship" .)

I've seen this clue in the Sydney Morning Herald.

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