An astronomical giant (5)
I believe the answer is:
orion
'giant' is the definition.
(thesaurus)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for orion that I've seen before include "Constellation with seven bright stars" , "One who would hunt" , "Greek hunter of myth or bright constellation" , "Great hunter of Greek myth, and a constellation" , "Rigel constellation" .)