Upstanding individual aboard high-speed train (6)
I believe the answer is:
honest
'upstanding' is the definition.
(I've seen this in another clue)
'individual aboard high-speed train' is the wordplay.
'individual' becomes 'one' (an individual is one person).
'aboard' indicates putting letters inside.
'high speed train' becomes 'hst'.
'one' inserted within 'hst' is 'HONEST'.
(Other definitions for honest that I've seen before include "Free from guile" , "Upright and fair" , "Honorable and sincere" , "Objective mediator" , "Direct" .)