This Miskatonic Horror Mythos card instructs the first player to roll 2 dice. The following sentence states:- "If the sum of the result is 7, each investigator with no Clue tokens gains 1 Clue token and loses 1 Sanity."

All of the 4 investigators in this game have at least 1 Clue token. If a 7 is rolled, do they still lose the 1 Sanity? in other words, is the second part of this sentence after the word "and" independent of the first part or conditional upon the first part?

Both losing Sanity and gaining the Clue are dependent upon having no Clues. For losing sanity to be independent the effect would have to be worded more like:

" If the sum of the result is 7, each investigator with no Clue tokens gains 1 Clue token. Then each investigator loses 1 Sanity. "

or:

" If the sum of the result is 7, each investigator loses 1 Sanity. Then each investigator with no Clue tokens gains 1 Clue token. "

At this time it would be somewhat ambigous if the second effect is dependent upon rolling 7, but I'm too tired to cross that hurdle.

Either way " gains 1 Clue token " and " loses 1 Sanity " are bundled into one package by the " and " between them.

The way I'd read it is that each investigator with clues is unaffected whereas each investigator without any clues gains a clue and loses 1 Sanity. Thus, in your example above, I'd suggest that no-one loses Sanity, nor do they gain any clues.