also: いちにかかって
expression
depends entirely on; hinges on
Used to emphasize that the outcome or success of something is completely determined by a single factor. Often followed by ～かどうか or a noun phrase.
成功するかどうかは、君の努力一つにかかっている。
Whether we succeed or not depends entirely on your effort.
合否は最後の面接一つにかかっている。
Whether he passes depends entirely on the final interview.
From 一つ (one thing) + に (particle) + かかって (te-form of かかる, 'to depend on'). Literally 'hanging on one thing', it became a set phrase meaning 'depends entirely on'.