It says in SA YD Siman 98 Sif 4 that even when you have shishim in a tavshil c’neged the issur (cheilev) you still must add water in order the the cheilev should float to the top and can be removed. If such a shailo happened on Shabbos what should one do to avoid shailos of borer…
(adding the water, taking out the issur, etc.)
If the fat is a separate entity to the liquid, which it presumably is if water can raise it to the top, then there is no problem with adding water: Raising a separate entity within a liquid is not an act of borer.
Taking out the fat will depend on the dispute between the Taz and other poskim (Maharit and others) concerning a mixture of solids and liquids.
According to many poskim, this does not constitute a “mixture,” and there is no problem in taking out the solid from the liquid. The Taz rules that there is a problem of borer, and the Mishnah Berurah mentions his opinon, writing that some of the soup should be taken out with the fly.
The same procedure should be used here.