Big name in astrophysics (4)
I believe the answer is:
neil
'big name in astrophysics' 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 "-- Sedaka, US singer" , "- Armstrong, first man to set foot on the Moon" , "-- Armstrong, Apollo astronaut" , "One of 14 DIAMONDS] 's" , "1969 moon walker" .)