Did rail, northbound, for free (7)
I believe the answer is:
deliver
'free' is the definition.
(delivering is a kind of freeing)
'did rail northbound' is the wordplay.
'did rail' becomes 'reviled' ('revile' can be a synonym of 'rail').
'northbound' shows that the letters should be reversed in order.
'reviled' written backwards gives 'DELIVER'.
'for' acts as a link.
(Other definitions for deliver that I've seen before include "what the errand boy does" , "ship" , "Hand over (to someone)" , "Bring (letters, etc)" , "Supply, hand over" .)