Approach getting former rail union down? (3-2)

I believe the answer is:

run-up

I'm a little stuck... Click here to teach me more about this clue!

'approach' is the definition.
(I've seen this before)

I cannot understand how the rest of the clue works.

(Other definitions for run-up that I've seen before include "Cricketing approach?" , "Approach - preliminaries" , "Preliminary period" , "Bowler's approach?" .)

I've seen this clue in The Times.

Want a hint initially instead of a full solution? Install my app