Men on adventure rising up, hiding immediately (4)
I believe the answer is:
anon
'immediately' is the definition.
(anon can mean soon or promptly)
'men on adventure rising up hiding' is the wordplay.
'hiding' indicates a hidden word.
'rising up' says the letters should be written backwards (in a down clue, letters go up).
'nona' can be found hidden inside 'men on adventure'.
'nona' in reverse letter order is 'ANON'.
(Other definitions for anon that I've seen before include "In a little while (informal)" , "Nameless (abbr.); soon" , "Soon; unattributed" , "Shortly; unknown" , "Of unknown identity, in short" .)