Critically analyse constant astronomical measurement (6)
I believe the answer is:
parsec
'astronomical measurement' is the definition.
(astronomical unit of distance)
'critically analyse constant' is the wordplay.
'critically analyse' becomes 'parse' (to parse is to analyse or break down a sentence).
'constant' becomes 'c' (mathematical abbreviation).
'parse'+'c'='PARSEC'
(Other definitions for parsec that I've seen before include "Long way" , "Unit of astronomical length" , "it takes years to get this far" , "unit for measuring distances" , "fair distance" .)