Astrophysicist — deGrasse Tyson (4)
I believe the answer is:
neil
'astrophysicist degrasse tyson' 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 walk on the moon" , "- Simon, US dramatist (Barefoot in the Park)" , "- Armstrong, first man to set foot on the Moon" , "1969 moon walker" .)