Many may have been a star! (5)
I believe the answer is:
deneb
'star' is the definition.
(star of the constellation Cygnus)
'many may have been a' is the wordplay.
I cannot quite see how this works, but
'many' could be 'd' (Roman numeral for 500) and 'd' is located in the answer.
an anagram of 'been' is 'eneb' which is located in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for deneb that I've seen before include "Be Ned (anag) - brightest star in Cygnus" , "First magnitude star in Cygnus" , "The star Alpha Cygni" , "Bright star; be end (anag.)" , "Bright star in Cygnus" .)