Celestial radio source (6)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">celestial radio source</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for pulsar that I've seen before include "Rapidly rotating star, emitting electromagnetic waves very quickly" , "Heavenly emitter of radio waves" , "A source of space of radio signals" , "Small and extremely dense star" , "Star producing radio waves" .)