1. nothing blocking your view, nothing in your way
a clear line of sight/thought/communication
To have a 'clear line' means to have a pathway or connection to something or someone which is not obstructed in any way. You could have a 'clear line of communication' meaning you are directly communicating with the person you need to talk to. A 'clear line of sight' meaning there is nothing obstructing your view from one place to another, or a 'clear line of thought' whereby your thoughts are focused. These are idiomatic phrases used in professional and social contexts.