。
She'll probably have to support herself.
Literal
She [topic-は] self-support do-must [probably-だろう].
自活 (じかつ, 'self-support, supporting oneself financially') is a compact Sino-Japanese compound — 自 ('self') + 活 ('live, support life') — that captures in two characters what English needs a whole verb phrase for. Becomes a verb with する. ~なければならない is the standard obligation pattern, and the sentence-final だろう ('probably, it seems likely') tones the obligation down from a stark statement to a tentative prediction. Together: a soft forecast of necessity.