Clean Intelligence?

Uncorrupted. Accurate.

"satellites, listening devices and other interception tools" allow the US to 'eavesdrop' on the Russians - to listen-in to conversations, even from thousands of miles away.
'dirty intelligence' refers to 'playing dirty', ' dirty tactics' to get information, such as sending spies into Russia, and planting 'moles' in key government departments.

Back in the day when homosexuality was illegal, a homosexual public servant could find themselves being blackmailed by a foreign power.

In the world of international espionage, "clean" means unknown to other intelligence services. So, if you get information about other nations, and they don't know you have obtained it, that is "clean intelligence." If you are able to intercept their signals (radio, telephone, telegraph, fax, internet, etc.--known as "sigint," for signals intelligence) without them knowing, that is clean intelligence. If you have agents in place in a foreign country that they don't know about, unless and until they get "burned," that is clean intelligence. That is known as "humint," meaning human intelligence, and it is often disregarded as a source of clean intelligence because your enemies may know of such agents, but leave them in place in order to feed disinformation to you. I have no idea if the author you are quoting is using the term in this sense.

Reading your passage again, i'm almost certain that the author is using the term properly. Signals intelligence is most likely to be clean intelligence, because the enemy cannot tell that you have intercepted it. Ironically, an elementary school teacher in England and his class of adolescent punters predicted the invasion of Afghanistan by the Soviets while the rest of the West was trying to analyze a bewildering flurry of rapid events in 1978. They simply intercepted and decoded Soviet satellite traffic--they used sigint, clean intelligence.

Thanks!