Armstrong on the moon (4)
I believe the answer is:
neil
'armstrong on the moon' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for neil that I've seen before include "Boy's name" , "Boy" , "One of 14 DIAMONDS] 's" , "- Armstrong, first man to walk on the moon" , "1969 moon walker" .)