i think its hard to have a clear cut sense of whats correct and whats not. I came across books on sampling and stuff, its either the book i read suck or that its a grey area coz the chapter which touch on clearance, doesnt really seem in depth and giving a clear sense of how much is too much and seeking clearance is the right thing, to prevent of any problem later.
Anyway, i went googling around and found these, theres quite a number of info on digital sampling. Most of it are really long and go into some law thingy which are too much to read for fun
still, interestingly, most of the early folks who bought sampling to another level by making new music thru it, doesnt really have clearance for the phrase they took from another piece of music.