Astronomical distance unit (6)
I believe the answer is:
parsec
'astronomical distance unit' is the definition.
(I've seen this in another clue)
This is all the clue.
(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)" .)