Hook, assuming you are hearing that correctly, is either a VOR nearby the airport or an intersection (which would then be spelled with five characters). "Direct" is just the instruction "fly directly to," so the entire phrase would be "fly DIRECTly to {VOR/Intersection} HOOK."
Now, where is this navaid/fix? I took a look at the approach plates for KAUS and did not see any navaid/fix that remotely sounds like HOOK. I am still looking on some charts to see if this is part of a departure procedure or a nearby VOR.