It was actually rerecorded song from Sound of Sunforest, 1969; film version differs from soundtrack version) Sound of Sunforest is the first and only studio album by psychedelic folk group Sunforest.
Two tracks from the album, “The Overture to the Sun” and “Lighthouse Keeper” were re-recorded by the band to be used in Stanley Kubrick’s A Clockwork Orange.
The soundtrack album credits the tracks to Terry Tucker and Erika Eigen, respectively. “Lighthouse Keeper” was also featured in television series The Maxx. (source Wikipedia)