One who's stepping out (11)
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" .)