Sagittarius aims at this viewer of Taurus (8)
I believe the answer is:
bullseye
'taurus' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.
'sagittarius aims at this viewer' is the wordplay.
'sagittarius aims' becomes 'bulls' (I can't explain this - if you can you should believe this answer much more).
'at' is a charade indicator (letters next to each other).
'this viewer' becomes 'eye' (organ which sees or views).
'bulls'+'eye'='BULLSEYE'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for bullseye that I've seen before include "A sucker" , "something to be aimed at" , "What may regard rival stock" , "window" , "Dead centre - Bill Sikes's dog - sweet" .)