Enter, for one (3)
I believe the answer is:
key
'enter for one' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for key that I've seen before include "Essential; solution" , "One that locks" , "Eg, Ctrl, Alt or Esc" , "Typewriter button, isle" , "Scale system for music" .)