Joy to behold everything the viewer is aware of (6)
I believe the answer is:
eyeful
'joy to behold everything the' is the definition.
I can't judge whether this definition defines the answer.
'viewer is aware of' is the wordplay.
'viewer' becomes 'eye' (I've seen this before).
'is aware of' becomes 'ful' (I can't explain this - if you can you should believe this answer much more).
'eye'+'ful'='EYEFUL'
Can you help me to learn more?
(Other definitions for eyeful that I've seen before include "A (lovely) sight to behold" , "As much as can be taken in by a look" , "Here butcher's" , "Something worth seeing" , "Attractive sight" .)