Main line transport? (4,5)
I believe the answer is:
boat train
'main line transport?' is the definition.
'boat train' can be an answer for 'transport?' (boat train is a kind of transport). I'm not certain of the 'main line' bit.
'line transport?' is the wordplay.
I cannot quite see how this works, but
'line' could be 'train' (train is a kind of line) and 'train' is present in the answer.
'transport?' could be 'boat' (boat is a kind of transport) and 'boat' is located in the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for boat train that I've seen before include "connecting transport" , "that runs down track to harbour" , "One may be heading for a port" , "Transport to/from docks" .)