noun
source of anxiety; reason for concern
Refers to a specific thing, situation, or thought that causes worry or unease. Often used when identifying what is making someone anxious.
彼の健康状態が不安の種だ。
His health condition is a source of anxiety.
将来のことが不安の種になっている。
The future has become a reason for concern.
The abstract feeling of anxiety; 不安の種 is the concrete cause of that feeling.
Compound of 不安 (anxiety) + の (possessive particle) + 種 (seed, source). Literally 'seed of anxiety', a common metaphorical pattern in Japanese for the origin or cause of something.