。
She has a thorough grasp of the heart of the issue.
Literal
She [topic-は] that issue [genitive-の] core [object-を] well is-understanding.
核心 ('core, heart, crux') is a stronger word than the everyday 中心 ('center'); it implies the essential, decisive part of something, as in 核心を突く ('strike at the heart of the matter'). 理解する ('to understand') in the ~ている form describes the resulting state of having understood, rather than the moment of understanding — the same stative pattern that gives 知っている and 持っている their everyday meanings. Pairs naturally with よく ('well') for thorough comprehension.