The most common and most likely answer to this crossword clue is the 8 letter word EYEPATCH
I've seen this crossword clue in The Daily Telegraph
E
Y
E
P
A
T
C
H
Buddy explains!

The straight part
We know that "cover for viewer" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case cover for viewer = eyepatch.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does EYEPATCH mean?
EYEPATCH noun- An opaque patch worn so as to cover one eye in order to protect or conceal it, or to treat amblyopia in children.