Cheapest class of air or rail travel (7)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
economy
(Other definitions for economy that I've seen before include "Me coy? No! (anag)" , "Financial prudence" , "Careful use of resources" , "Thrift - national production, distribution and consumption" , "Efficient use of resources" .)