One banked on needing loco — did one? (6,6)
I believe the answer is:
engine driver
'one' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'banked on needing loco did one?' is the wordplay.
I cannot quite see how this works, but
an anagram of 'needing' is 'engined' which is within the answer.
'one?' could be 'i' (Roman numeral) and 'i' is found within the answer.
This may be the basis of clue (or it may be nonsense).
(Other definitions for engine driver that I've seen before include "Railway employee" , "loco?" , "Footplate man" , "Railway worker" , "Once the dream career of schoolboys?" .)