Viewer by red building that's ugly (7)
I believe the answer is:
eyesore
'that's ugly' is the definition.
I can't judge whether this defines the answer.
'viewer by red' is the wordplay.
'viewer' becomes 'eye' (organ which sees or views).
'by red' becomes 'sore' (I can't explain this - if you can you should give a lot more credence to this answer).
'eye'+'sore'='EYESORE'
'building' acts as a link.
Can you help me to learn more?
(Other definitions for eyesore that I've seen before include "An unsightly thing" , "Something ugly and offensive" , "ugly feature" , "pigsty" , "Monstrosity" .)