I personally detest the modern scientific thought-frame that its just "a clump of cells". That terminology seems Orwellian to me, and is a slippery slope to quickly dehumanize the subject.
I'm in favor of a choice to abort in special circumstances such as rape or mental defects, but then again that simply opens the door to more loosely defined slopes. Its like...where do we draw the line? Its either 100% yes or 100% no.
I think the point of no return should be after there's some kind of brain activity, which is usually 5-6 weeks. Before that, it would be more like pulling the plug on someone who's braindead than it is infanticide. I would also say that it's justified to prevent the child from being born with a life-long illness like downs-syndrome, or especially something more severe that we wouldn't be able to treat.
However, no matter what we think people are going to be morally flexible about this. Same thing with eating meat. Morally, most of the animals we eat do have some form of sentience and killing them is pretty much definitely murder, however, fuck you I don't care I want my steak. People are going to apply the same mindset to abortion, and I personally think anyone who claims one is murder but the other isn't is a hypocrite. The life of a fetus can be dismissed easily for the same reasons as the life of a cow or a pig or a lamb, as well as defended for the same reasons. It is essentially the same argument, really.
What I look forward to, in both cases, is when slaughtering animals and killing fetuses does not become necessary to produce meat or terminate a pregnancy. Lab-grown meat will be a common thing in our lifetimes, as will artificial wombs. Once these are more common, it'll probably be easier for people to accept the blatant and needless immorality of abortion or eating meat, since there are now perfectly moral alternatives that provide the same result.