Is it possible to see mercury transit "clearly" by the naked eye?
I did see the Venus transit before the Sun in 2004. 

I used no telescope, but of course I used proper solar eclipse glasses to protect my eyes. The black circle was small but clearly visible.

But Mercury is much smaller, the transit is not visible without a telescope. See Wikipedia.

Here an image of a Venus transit:



Here the much smaller Mercury:



The dark spots to the right and left are sunspots, Mercury is the very tiny black spot below the center of the Sun.

Images from Wikipedia.