Star starts to regret it backstage (5)
I believe the answer is:
rigel
'star' is the definition.
(star in the constellation Orion)
'starts to regret it backstage' is the wordplay.
'starts to' suggests taking the first letters.
'backstage' becomes 'gel' (I can't justify this - if you can you should believe this answer much more).
The initial letters of 'regret it' is 'ri'.
'ri'+'gel'='RIGEL'
Can you help me to learn more?
(Other definitions for rigel that I've seen before include "Orion's brightest star" , "Brightest star in Orion" , "shiner" .)