The distance is simply astronomical (6)
I believe the answer is:
parsec
'the distance' is the definition.
(I've seen this before)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for parsec that I've seen before include "Unit for measurement of distance of stars" , "Large unit of distance" , "Unit of astronomical distance - scrape (anag)" , "Astronomical length - scrape (anag)" , "it takes years to get this far" .)