I've just spent some time re-watching that 'hip-drop' tackle.
I'm definitely no expert but it wasn't a hip-drop as much as it was a regular tackle where the ball carrier beats first contact. Crossland looks to lose his footing, Cleary runs past the point of initial contact and Crossland manages to hold on, just, landing, just, on Clearys foot/ankle.
I'm sure there will be unlimited technical analysis of this tackle over the next 48 hours but i gotta say we see this type of thing dozens of times a game. The only reason we're talking about it is because of the injury and the apparent resemblance to a hip drop