Old fashioned engine driver (5)

I believe the answer is:

steam

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'engine driver' is the definition.
(a steam engine is powered by steam)

I don't understand the rest of the clue.

(Other definitions for steam that I've seen before include "enthusiasm" , "Vapour; energy" , "Type of power" , "Vaporous exhalation" , "Water at boiling temperature diffused in the atmosphere" .)

I've seen this clue in the Evening Standard.

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