Temptress loosening reins? (5)

I believe the answer is:

siren

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'temptress' is the definition.
(I know that temptress can be written as siren)

'loosening reins?' is the wordplay.
'reins' can be anagrammed to 'SIREN'.
However, I am not clear how the anagram is indicated.

Can you help me to learn more?

(Other definitions for siren that I've seen before include "Hooter - dangerous woman?" , "She tempts" , "Temptress - makes a loud noise!" , "Vamp; alarm" , "Bewitching singer" .)

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