Fast train (7)
I believe the answer is:
express
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">fast train</span>' is the definition.<br/ ><span class="explanation-format__explanation">(an express train)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for express that I've seen before include "Show" , "Utter - direct" , "Transport" , "Explicit" , "Specific; going at speed" .)