Not by wrestling but by clinging is referring to Jacob wrestling with the angels all night until he finally took hold of him and wouldn’t let go until the angel blessed him.
Both wrestling and clinging take an effort. Both have action in mind, either mine that is by nature Satan's or the Lord's. The determining factor is as far as we are concerned our will. That's why the Lord Jesus decided in the ultimate hour of absolute dread "Not My will be done but Thine." Our clinging says: "I will that Your will be done in my life. Help me to bend my will absolutely to Yours because I absolutely trust You."
Thank you, Isaiah. It wasn't clear to me what the wrestling meant until I read your comment! Praise the Lord that we can help each other come closer to our Lord by sharing our insights and our love for Him.
The writer of this hymn portrays oneness with Christ in terms of deep spiritual intimacy and dependence on Him in every single way. To "wrestle" here implies the energies of the flesh, as opposed to those of the mingled spirit. To "lean" is to resign all our efforts and resources to take Him as everything to us. Surely this requires us to yield to Him completely. But for this to happen we need to die to self, and have our natural energy, the strongest part of our natural being, made fundamentally "lame" so that we with our Jacobite nature may become the transformed and matured Israel where our very being so gained and saturated by the Lord is "victory".