That depends. Are we considering only rational numbers to be fractions? If so, then 1 is not a fraction of pi but pi/4 is a fraction of pi. Typically, our day-to-day understanding of the word "fraction" is that the fraction is smaller than the whole, so 1 is a fraction of pi, but neither 5 nor pi is a fraction of 1.
You use "de" for quantity of a specific thing. The word "la" usually follows "de" when its used this way, but in this example you don't need it since you don't put "la" in front of a cardinal number.
Read "More about Da/De" at the bottom of this page: https://www.duolingo.com/skill/eo/Da_De.
x is a fraction of y if x/y is rational, that is, if x/y = p/q, where p and q are integers (the integers are 0, 1, -1, 2, -2, 3, -3, ...).
Thus pi/4 is a fraction of pi since (pi/4)/pi = 1/4.
Technically 6 is a fraction of 5/2, since 6/(5/2) = 12/5, a rational number. Zero is a fraction of any non-zero number x, since 0/x=0, a rational number.
In common usage though, when x is said to be a fraction of y, one expects x/y to be a rational number between 0 and 1.
So one says 1 is a fraction of 6, but 6 is not a fraction of 1. Note this is "common usage". Technically, 6 is a fraction of 1.