Old engine driver? (5)
I believe the answer is:
steam
This clue might be a double definition.
'old' is the first definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'engine driver' is the second definition.
(a steam engine is powered by steam)
Can you help me to learn more?
(Other definitions for steam that I've seen before include "Speed" , "Water vapour; impetus" , "Form of water" , "Vaporous exhalation" , "Energy (colloq.)" .)