Police force is back for support (4)

I believe the answer is:

stem

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'for support' is the definition.
(I've seen this before)

'police force is back' is the wordplay.
'police' becomes 'met' (London Metropolitan Police).
'force' becomes 's' (I can't justify this - if you can you should give a lot more credence to this answer).
'is back' says the letters should be written backwards.
'met'+'s'='mets'
'mets' reversed gives 'STEM'.

Can you help me to learn more?

(Other definitions for stem that I've seen before include "Ski turn and plant support" , "Tube" , "Type of cell" , "section of pipe" , "Vertical part of flower or drinking glass" .)

I've seen this clue in The Times.

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