Rotating neutron star (6)
I believe the answer is:
pulsar
'rotating neutron star' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for pulsar that I've seen before include "Rapidly rotating star, emitting electromagnetic waves very quickly" , "Rapidly fluctuating star" , "Radio-emitting star" , "Star producing radio waves" , "A source of space of radio signals" .)