Check trainers have been sent (8)

I believe the answer is:

restrain

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'check' is the definition.
(synonyms)

'trainers have been sent' is the wordplay.
'trainers' anagrams to 'RESTRAIN'.
However, I'm unsure how this is indicated.

Can you help me to learn more?

(Other definitions for restrain that I've seen before include "Control - check" , "Stop" , "Keep under control" , "bar" , "stay" .)

I've seen this clue in The Mirror.

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