Crossword Genius

Fast train clearly indicated (7)

Ross

I believe the answer is:

express

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'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" .)

I've seen this clue in The Telegraph.
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