Clue: 8 25% reduction of electric power to audio visual aid (6)
Answer: VLTAVA
Explanation: Definition: 8
V[O]LT + AVA – the Volt isn't a measure of power in the technical sense, but colloquially I think it's OK. However I'm not keen on "25% reduction" meaning "remove one letter from inside the word", though I suppose it's accurate enough. I know the name of this Czech river (also called the Moldau) from the music of the same name by Smetana, but I pity anyone who doesn't know it trying to solve on the bus..