Crossword Genius

One who's stepping out (11)

Ross

I believe the answer is:

sleepwalker

'one' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.

I cannot understand how the remainder of the clue works.

(Other definitions for sleepwalker that I've seen before include "We paler elks (anag)" , "Somnambulist" , "One stepping out" , "Noctambulist" .)

I've seen this clue in The Independent.
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