As a witness, be careful! (5)

I believe the answer is:

watch

I'm a little stuck... Click here to teach me more about this clue!

'as' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for watch that I've seen before include "Observe - guard" , "See 21" , "period of duty" , "Observe - timepiece" , "Look at attentively" .)

I've seen this clue in the Evening Standard.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.