Analyse speed of light, getting astronomical distance (6)
I believe the answer is:
parsec
'astronomical distance' is the definition.
(I've seen this before)
'analyse speed of light getting' is the wordplay.
'analyse' becomes 'parse' (parsing is a kind of analysing).
'speed of light' becomes 'c' (I've seen this in other clues).
'getting' means one lot of letters go next to another.
'parse'+'c'='PARSEC'
(Other definitions for parsec that I've seen before include "Three+ light-years" , "Unit of distance in space" , "Long way" , "vast length" , "Enormous distance" .)