Enormously large (12)
(Other definitions for astronomical that I've seen before include "Of the study of stars" , "Really, really big" , "Tremendous" , "Inconceivably large" , "Absolutely huge, in a heavenly way" .)
'enormously large' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for astronomic that I've seen before include "Sky-high (colloq.)" , "whacking" , "Immense" , "Incredibly large - so romantic (anag)" , "Relating to science" .)