Is it possible to use part of my HD for Time Machine?

Time machine, internal, and external

Normally, I'd say, don't back up to the same physical drive; if it dies, you'll lost just as completely as if you had no backup at all.

But your situation is a tad more complicated, as you say you're backing up to an external drive, too. If that external drive were big enough - say 1.5 times the size of the internal, or more - I'd say switch tactics and use the external as the Time Machine target. But I understand your desire to have two backups, so I'd say one strategy is about as good as the other.

Personally, I do it as follows:

1) one partition on internal drive
2) one larger TM target external drive
3) one other external for clone backups, just in case
4) coming soon - a Time Capsule to use as another Time Machine target

I'll undoubtedly change my mind and mess with things further as time goes on, but that's just me.

You can't have too many backup devices, unless things are so complex for you that you get confused about what is where; that can cause you to accidentally overwrite new stuff with old. Don't ask how I know this.

Bottom line: disk space is cheap; buy more and be safer.

Tom