St. John the Apostle's focus on the light of Christ makes him a perfect intercessor for clarity.
In the first few verses of his gospel, St. John dwells upon Jesus being a “light” in the world.”
In him was life, and the life was the light of men. The light shines in the darkness, and the darkness has not overcome it … The true light that enlightens every man was coming into the world. (John 1:4-5, 9)
This emphasis is why St. John is frequently invoked for clarity in both mind and soul.
Here is a brief prayer partially based on the collect for the Mass celebrated on his feast day that asks God to shed his light upon us all, through the intercession of St. John the Apostle.
Merciful Lord, we ask you to cast your bright beams of light upon your Church, that, it being enlightened by the doctrine of your blessed apostle and evangelist Saint John, may so walk in the light of your truth, that it may at length attain to the light of everlasting life, through Jesus Christ our Lord. Amen.