Fast train clearly indicated (7)
I believe the answer is:
express
'fast train' is the definition.
(I've seen this before)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for express that I've seen before include "Utter; speedy" , "Put into words - fast" , "High-speed; specific" , "Specific; going at speed" , "air" .)