Hard nuts cracked by unknown group of stars (5)
I believe the answer is:
hydra
'group of stars' is the definition.
(constellation)
'hard nuts cracked by unknown' is the wordplay.
'nuts' indicates anagramming the letters (nuts can mean crazy).
'cracked by' is an insertion indicator (to crack can mean to create a divide in).
'unknown' becomes 'y' (algebra).
'hard' is an anagram of 'hdra'.
'hdra' enclosing 'y' is 'HYDRA'.
(Other definitions for hydra that I've seen before include "Small red-berried tree" , "Mythic monster with nine heads" , "Mythological monster with nine heads" , "Proliferating-head monster" , "Many-headed serpent" .)