Ten trains running without delay (9)
I believe the answer is:
instanter
'without delay' is the definition.
(I've seen this before)
'ten trains running' is the wordplay.
'running' indicates an anagram (the letters run around to different places).
'ten'+'trains'='tentrains'
'tentrains' anagrammed gives 'INSTANTER'.
(Other definitions for instanter that I've seen before include "On the spot" , "in court, immediately!" , "without delay" , "without hesitation" .)