A letter opener, perhaps (5)

I believe the answer is:

steam

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'a letter opener' is the definition.
(I've seen this before)

I don't understand how the remainder of the clue works.

(Other definitions for steam that I've seen before include "Water-vapour" , "Water at boiling temperature diffused in the atmosphere" , "It's water mist, mates" , "letter opener?" , "A lot of hot air" .)

I've seen this clue in the Evening Standard.

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