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が気がかりだ

… is a concern

→ が
phrase Rare #34,680
… is a concern The thing before が is what the speaker is worried about; the person feeling worried is usually understood from context.
JA: 明日の天気が気がかりだ。
EN: I’m concerned about tomorrow’s weather.

彼の安否が気がかりだ。

I'm worried about whether he's safe.

日本の未来より、お前の将来の方が気がかりだ。

I'm more worried about your future than Japan's.

私は将来のことが気がかりだ。

I'm worried about the future.