You may travel in one but not fast! (6,3)
I believe the answer is:
dining car
'not fast' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'you may travel in one' is the wordplay.
I cannot quite understand how this works, but
'in' is within the answer.
'one' could be 'i' (Roman numeral) and 'i' is present in the answer.
This explanation may well be incorrect...
'but' is the link.
Can you help me to learn more?
(Other definitions for dining car that I've seen before include "Restaurant area on train" , "Railway coach serving food" , "Railway carriage with a restaurant" , "fast food here" , "meals on wheels provider" .)