The key to this riddle is only for you
Below are instructions, above this the clue
First strike the one near the head of a year
Then remove he who begins a cheer
Next take away the end of a tunnel
Then let us go to solve this puzzle
What you first took you must now take again
With three you are left, but fret not dear friend
Fattest to front and thinnest to rear
Add in two "eyes" and all becomes clear
Remark: The instructions in this riddle are quite literal.
The answer is XVIII, or 18 in Roman numberals.
The key is "only for you" - EXCLUSIVE. Remove "the one near the head of a year" - E. Remove "he who begins a career" - C. Remove "the end of the tunnel" - L. Remove "us" - U and S. Remove again E. Now you are left with X, I, V. Arrange them "fattest to front and thinnest to rear" - X > V > I. Add "two 'eyes'" - I and I, so you get XVIII=18.