Backed Irish setter producing big blue shiner (5)
I believe the answer is:
rigel
'big blue shiner' is the definition.
(brightest star in Orion)
'backed irish setter' is the wordplay.
'backed' is a reversal indicator.
'irish' becomes 'IR' (abbreviation).
'setter' becomes 'gel' (something which sets).
'ir' backwards is 'ri'.
'ri'+'gel'='RIGEL'
'producing' acts as a link.
(Other definitions for rigel that I've seen before include "Brightest star in Orion" , "shiner" , "Orion's brightest star" .)