Astronomical distance unit (6)
I believe the answer is:
parsec
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">astronomical distance unit</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 parsec that I've seen before include "far in the distance" , "fair distance" , "nineteen million million miles" , "Three+ light-years" , "Astronomical length - scrape (anag)" .)