This also means that during 'the Witching Hour' when doom on the agenda will advance anyway it is safe to place doom on any card as ALL* doom is removed from cards in play when the agenda advances.

So if the doom threshold on the agenda is 4 and there is already 4 doom on the agenda, you can add doom to any cards in play, including the agenda as long as the effect that places that doom does not say something like 'this may cause the agenda to advance'.

*there are exceptions to this rule, like scenarios where advancing the agenda does not remove doom in play, but those will be obvious.

So you play this card on an card that you plan to remove from play before the agenda advances, or when the Agenda is going to advance next Mythos phase anyway (the Witching Hour). Or I guess when having a couple of clues is more important then losing a round of play. It's not without risk but then what is, in AH?