Some Circuits, like the 6th Circuit publish pattern jury instructions defining reasonable doubt. The 4th Circuit in 1999, U.S. v. Walton, et al, decided that it was better if the court did not define the term in criminal cases, but left it to the jury to decide what it meant. So, that is what I expect Judge Ellis to tell them. This will not resolve whatever confusion may have prompted the question, which can only benefit Manafort.
What Judge Ellis answered (according to the Twitter feed of a reporter covering the trial) was that reasonable doubt means “doubt based on reason,” and that the government is not required to prove guilt beyond “all possible doubt.”