Instruments sending sound up in urgent message (7)

I believe the answer is:

speedos

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'instruments sending' is the definition.
I don't know anything about this answer so I cannot tell whether it can be defined by this definition.

'sound up in urgent message' is the wordplay.
'sound' becomes 'deep' (synonyms).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'in' indicates putting letters inside.
'urgent message' becomes 'SOS' (an SOS message sent in an emergency).
'deep' backwards is 'peed'.
'peed' put within 'sos' is 'SPEEDOS'.

Can you help me to learn more?

(Other definitions for speedos that I've seen before include "Car velocity displays" , "Swimmers" , "Dashboard instruments (colloq.)" .)

I've seen this clue in The Times.

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