Astronomical distance (6)

I think it could be either:

Which length description are you looking for?:

5-4

parsec

'astronomical distance' is the definition.
(I've seen this before)

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" .)

I've seen this clue in the Penny Press, The Guardian, The Mirror and the Sydney Morning Herald.

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