Final answer:
The sitar is the Indian instrument featured on George Harrison's song 'Within You Without You'. The song showcases a fusion of Eastern and Western music, prevalent in The Beatles' later works, specifically on the 'Sgt. Pepper's Lonely Hearts Club Band' album.
Step-by-step explanation:
The important Indian instrument overdubbed on George Harrison's song 'Within You Without You' is the sitar. The sitar is a plucked stringed instrument, used mainly in Indian classical music and became popular in the western world through the influence of musicians like George Harrison of The Beatles. Harrison incorporated the sitar into his music, creating a fusion of Eastern and Western musical styles, during the 1960s which contributed significantly to the unique sound of some Beatles' songs, including 'Within You Without You' from the album Sgt. Pepper's Lonely Hearts Club Band.