The most common and most likely answer to this crossword clue is the 6 letter word VIEWER
I've seen this crossword clue in Independent
V
I
E
W
E
R
More clues leading to the result VIEWER
- Spectator we found absorbed by a German quartet
- Struggle with editor’s extremes in the Spectator
- Person striving to comprehend compilers in The Observer
- Inspector gives opinion with hesitation
- One observes contrary directions in Mozart’s quartet
- Endless French wine by the jug for 4
- One who sees six sheep north of river
- This person can provide a balanced review
- Spectator right behind six sheep
- Witness struggle over nude twerp
Buddy explains!

The straight part
We know that "Witness" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case witness = viewer.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 VIEWER mean?
VIEWER noun- Someone who views a spectacle; an onlooker or spectator.
- Someone who watches television.
- Any optical device used to view photographic slides.
- A program that displays the contents of a file.
- The manager of a colliery, who directs its workings and ventilation.