Punt ace, not the brightest (4)
I believe the answer is:
beta
'not the brightest' is the definition.
(second brightest star in a constellation)
'punt ace' is the wordplay.
'punt' becomes 'bet' (I've seen this in other clues).
'ace' becomes 'a'.
'bet'+'a'='BETA'
(Other definitions for beta that I've seen before include "Second letter of the Greek alphabet" , "Second-class" , "It's next after alpha" , "Letter before gamma" , "less than perfect mark" .)