Star cluster emitting light, extremely brightly first of all (5)
I believe the answer is:
celeb
'star' is the definition.
(I've seen this in another clue)
'cluster emitting light extremely brightly first of all' is the wordplay.
'first of all' says to take the initial letters.
The initial letters of 'cluster emitting light extremely brightly' is 'CELEB'.
(Other definitions for celeb that I've seen before include "Prominent figure" , "Famous person (colloq.)" , "Famous person (abbr)" , "popular star" , "Famous person (sl.)" .)