Old railwaymen climbing ladder (3)
I believe the answer is:
run
'ladder' is the definition.
('run' can be a synonym of 'ladder')
'old railwaymen climbing' is the wordplay.
'old railwaymen' becomes 'nur' (National Union of Railwaymen).
'climbing' says the letters should be written backwards (in a down clue, letters climb up).
'nur' in reverse letter order is 'RUN'.
(Other definitions for run that I've seen before include "Scarper" , "Enter a race" , "freedom of access" , "Flow; function" , "A single" .)