You will take claim on a monster if you make a claiming motion while someone in your party is at the top of the hate list, OR if the monster currently does not have an aggro/hate target.
This weekend I was in Thickets on my BLM, fighting other people for puks. One puk aggroed another party, so I "pulled" him with slow. That put me on top of the hate list, but since the list wasn't empty, it stayed white. Their bard then slept it. Since I was previously on top of the hate list, it stayed white. However, now their bard was on top of the hate list. I then cast Thunder3 on it. Since their bard was on top of the hate list when the spell landed, it stayed white, though Thunder3 put me on top of the hate list thereafter. I followed that up with a stun, which finally caused it to become claimed to me, and then a sleep to keep myself from dying while our PLD established hate on it.