Train sent off without delay (9)
I believe the answer is:
instanter
'without delay' is the definition.
(I've seen this before)
'train sent off' is the wordplay.
'off' indicates an anagram.
'train'+'sent'='trainsent'
'trainsent' with letters rearranged gives 'INSTANTER'.
(Other definitions for instanter that I've seen before include "without delay" , "On the spot" , "in court, immediately!" , "without hesitation" .)