US commando in false alarm (4)

I believe the answer is:

seal

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'us commando' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.

'in false alarm' is the wordplay.
'in' indicates a hidden word.
'SEAL' can be found hidden inside 'false alarm'.

Can you help me to learn more?

(Other definitions for seal that I've seen before include "Ratify" , "Close tightly - mammal" , "Secure; aquatic animal" , "Make airtight" , "stop leaks" .)

I've seen this clue in The Sun.

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