Relax, Stew! (6,4)

I believe the answer is:
simmer down
'relax' is the definition.
(I've seen this in another clue)
'stew' is the wordplay.
I cannot quite understand how this works, but
'stew' could be 'simmer' (I've seen this in another clue) and 'simmer' is found in the answer.
The remaining letters 'down' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
I've seen this clue in the Evening Standard.
Want a hint initially instead of a full solution?
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