There are good answers here already about how to directly either get out of that situation or address the issue with your partner, but I'm going to take a different approach and suggest a shift in your own thinking.
If your partner is trustworthy and faithful to you, then another woman hitting on him is not a threat to you or your relationship, and is really just an affirmation that your partner is desirable. So try to take it as a compliment!
Because my wife and I have a solid relationship (and did even back when we were just dating), she actually loves it when other women hit on me at parties. She finds it hilarious to see me try to steer them away from the flirtation (I can be a bit awkward in those situations), and I think she likes seeing confirmation that even though other people may be interested in me, my interests lie entirely with her. Same goes for when other men hit on her, she likes the attention and social affirmation, and when she's happy, I'm happy. We both know that at the end of the night it's the two of us who are going home together, and no amount of flirtation from others is going to change that.
On the other hand if your partner is not faithful to you, this will be a good situation for revealing that in them. If that turns out to be the case (or you already knew that to be the case), then in most situations it seems to me that you are better off without that person. Trying to fight against a partner's impulses for infidelity seems excruciating and stressful, definitely not worth it. Find a relationship in which you can feel safe and secure and you will save yourself a lot of grief in the future.