Astronaut Armstrong (4)
I believe the answer is:
neil
'astronaut armstrong' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for neil that I've seen before include "- Armstrong, first man to walk on the moon" , "-- Sedaka, US singer" , "-- Armstrong, Apollo astronaut" , "- Armstrong, first man to set foot on the Moon" , "Boy" .)