Secretive celebrity's large body not evident to viewers (4,4)
'secretive' is the definition.
(thesaurus)
I cannot understand how the remainder of the clue works.
(Other definitions for dark that I've seen before include "Black, perhaps - night" , "Sinister - evening" , "Nightfall; without light" , "Dimly lit" , "Obscure - sinister" .)
'large body not evident to viewers' is the definition.
(type of star emitting no light)
'secretive celebrity's' is the wordplay.
'secretive' becomes 'dark'.
'celebrity's' becomes 'star' (I've seen this before).
'dark'+'star'='DARK STAR'
(Another definition for dark star that I've seen is " Carpenter made this".)