Date one person regularly in travel firm (2,6)
I believe the answer is:
Here is my best explanation:
'date one person regularly' is the definition.
(I've seen this before)
'travel firm' is the subsidiary indication.
'travel' becomes 'go' ('go' can be a synonym of 'travel').
'firm' becomes 'steady' ('steady' can be a synonym of 'firm').
'in' is the link.
(Other definitions for go steady that I've seen before include "Have relationship as regular boy or girl friend - sounds stable", "Date one person regularly" and "Court romantically".)