（只有）一条缝
so that there is a very small space
She opened the door just a crack to listen to the conversation. 她把门打开一条小缝听人谈话。
We left the gate open just a crack, hoping the dog would return.
I like to leave my bedroom window open a crack at night.
The office door opened a crack, and a woman's face appeared.
She kept her visor open just a crack to keep it from fogging up inside.
See also: crack noun (NARROW SPACE)