He's among group of stars in reference book (9)
I believe the answer is:
thesaurus
'reference book' is the definition.
(I know that thesaurus is a type of thematic dictionary)
'he's among group of stars' is the wordplay.
'among' means one lot of letters goes inside another.
'group of stars' becomes 'taurus' (I can't justify this - if you can you should believe this answer much more).
'hes' placed into 'taurus' is 'THESAURUS'.
'in' acts as a link.
Can you help me to learn more?
(Other definitions for thesaurus that I've seen before include "Wordy gathering" , "Book of words with synonyms, antonyms etc" , "Reference book" , "Reference work" , "Book listing synonyms" .)