Athlete’s approach getting old rail union down? (3-2)
I believe the answer is:
run-up
'athlete's approach' is the definition.
'run up' can be an answer for 'approach' (I've seen this before). I'm unsure of the 'athlete's' bit.
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for run-up that I've seen before include "Cricketing approach?" , "Preliminary period" , "Bowler's approach?" , "Approach - preliminaries" .)