Astronomical distance (6)
'astronomical distance' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for parsec that I've seen before include "Immense unit of distance" , "Unit of astronomical length" , "three and a quarter twenty sevens [LIGHT YEARS]" , "nineteen million million miles" , "Three+ light-years" .)
'astronomical distance' is the definition.
(I've seen this before)
This is all the clue.