A clean mind is not an empty mind. Our mind can never be entirely empty of thoughts, feelings and ideas. Curating one’s mind requires experience and discernment.
Our mind (or dianous) is the instrumental part of our psyche. The highest part of our psyche is our intellect (or nous).
It is through our nous that we perceive and experience divine nature.
Our nous manages our dianous. When our nous is illumined by the divine grace of the Holy Spirit, then we manage our minds well.
Our nous is clarified by divine grace in worship and prayerful attention. When that happens, then we are able to manage ourselves with discernment and discretion in the field of our mental activity.
Our choices regarding what sensations to experience, activities to engage in, ideas to reflect upon, emotional responses to make, etc., will all become morally virtuous when guided by a godly light in our nous.
Then our mind will be a well-functioning tool, a cleaned instrument.