。
She fastened the necklace clasp.
Literal
She [topic-は] necklace [genitive-の] clasp [object-を] fastened.
留め金を留める shows the deep connection between 留める ('to fasten, to hold in place') and 留め金 ('a clasp, fastener') — the noun is literally a 'fastening-metal,' built from the verb's masu-stem 留め plus 金 ('metal'). This is a productive way Japanese builds names for tools and parts: the verb stem describes the tool's function. 受け皿 ('saucer,' from 受ける 'receive' + 皿 'plate'), 開け方 ('the way of opening'), 飲み物 ('drink,' literally 'drinking-thing') all follow the same logic.