Crossword Genius

Old railwaymen climbing ladder (3)

Ross

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" .)

I've seen this clue in The Independent.
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