Journey mostly at universal speed, leading to success (7)
I believe the answer is:
triumph
'to success' is the definition.
(I've seen this before)
'journey mostly at universal speed leading' is the wordplay.
'journey' becomes 'trip' (trip is a kind of journey).
'mostly' means to remove the last letter (most but not all of the word).
'at' says to put letters next to each other.
'universal speed leading' becomes 'umph' (I can't justify this - if you can you should give a lot more credence to this answer).
'trip' with its last letter removed is 'tri'.
'tri'+'umph'='TRIUMPH'
Can you help me to learn more?
(Other definitions for triumph that I've seen before include "Great victory or achievement" , "Exult in victory" , "Be victorious" , "former car manufacturer" , "Victory, win" .)