Crossword Genius

But it’s certainly no “express” (4,5)

Ross

I believe the answer is:

milk train

'express' is the definition.
(thesaurus)

I don't understand how the rest of the clue works.

(Other definitions for milk train that I've seen before include "early-morning rail service" , "early service" .)

I've seen this clue in The Times.
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