Has-been might show spirit (7)
I believe the answer is:
banshee
'spirit' is the definition.
(spirit in Irish mythology)
'has-been might' is the wordplay.
'has-been' anagrams to 'BANSHEE'.
But, I am not clear how this is indicated.
'show' acts as a link.
Can you help me to learn more?
(Other definitions for banshee that I've seen before include "Ominous wailing spirit" , "'Wailing woman, warns of impending death (7)'" , "no one wishes to hear" , "Death-portending spirit" , "Wailing woman from Gaelic myth" .)