Munchkin Combat Rules Clarification: Do you lose one-shot items played if later forced to run away
Once you play a card, that's it. It is no longer in your hand. You can't pick it back up and put it into your hand. From the Munchkin FAQ under Miscellaneous Questions (emphasis mine):

Q. If I announce I'm going to do something, am I locked into that decision or can I change my mind? For instance, I was a Wizard and declared that I was going to fight a monster, but then the monster was enhanced further. Can I change my mind and Charm it?
A. You're NEVER locked into a final decision unless a card or rule says so. (However, there are no &quot;take-backs&quot; once you have played a card or rolled a die.)

If you start to play a card and then immediately change your mind, then you haven't actually played the card, and you aren't locked into your decision. However, if you play your card, and then one of your friends plays an Enrage, then the game has clearly progressed beyond the point where you can change your mind.