Rush up to catch train starting to stop (5)
I believe the answer is:
deter
'stop' is the definition.
(I've seen this before)
'rush up to catch train starting' is the wordplay.
'rush' becomes 'reed' (I've seen this before**).
'up' is a reversal indicator (in down clue: letters go upwards).
'to catch' is an insertion indicator.
'starting' says to take the initial letters.
The first letter of 'train' is 't'.
'reed' backwards is 'deer'.
'deer' going around 't' is 'DETER'.
'to' acts as a link.
(Other definitions for deter that I've seen before include "Hinder, prevent" , "Frighten off; prevent" , "Discourage through fear of consequences" , "'Discourage, dissuade (5)'" , "Discourage or prevent from doing something" .)