。
She's still active at the top of her field.
Literal
She [topic-は] now-also front-line [at-で] active is-being.
今も pairs 今 ('now') with the additive も to mean 'now too, still even now' — implying the situation has continued from earlier. 第一線 ('first line, front line') is a metaphor borrowed from military deployment, applied broadly to any leading position in a profession or field. 活躍する ('to be active, to play an active role, to perform with distinction') is a standard verb in CVs and journalism, often used in retrospectives and career commentary. で here marks 'where' the action takes place — the field or arena.