"Is it covered by guarantee?"
Looks like accidental damage, which unfortunately is
not covered.
If you bought it this way, then yes it's covered.
This is NOT accidental damage. This is a known weakness in the design and many macbook pros exhibit this. I've had several of these out of the box like this and I've seen many more that developed it over time. Apple simply didn't reinforce that spot or glue it to the internal frame. When you use the computer and put pressure from above by placing your hands on the palmrests, this can happen - especially if the aluminum is misaligned to begin with.
Warranty!