Rail traveller with no ticket who gets in first (6,6)
I believe the answer is:
Here is my best explanation:
'rail traveller' is the definition.
The definition and answer can be both people as well as being singular nouns.
Maybe you can see an association between them that I can't see?
'no ticket who gets in first' is the subsidiary indication.
I cannot quite understand how this works, but
'no' could be 'n' (common abbreviation) and 'n' is present in the answer.
'who' could be 'dr' (from 'Dr Who', the famous TV series) and 'dr' is present in the answer.
'in' is present in the answer.
'first' could be 'i' (I in Roman numerals eg Elizabeth I) and 'i' is present in the answer.
This may be the basis of clue (or it may be nonsense).
'with' is the link.
(Other definitions for engine driver that I've seen before include "Railway employee", "Once the dream career of schoolboys?", "Railway worker", "Footplate man" and "loco?".)