Yes, the chance of keeping the partially-used clip is the only way. You can't carry this partial clip past the end of battle due to how item data is stored; the ammunition in the clip and the clip as an item that contains ammo only exist during a battle, after that they go back to being an abstraction in your stores.
Edit: Just to make sure you know, the chance to recover a clip only affects a single clip; when you have multiple weapons using the same ammo, at the end of battle all the leftover ammunition is added up together and then divided to make whole clips. You will get 19 out of 20 rifle clips back no matter whether you fire one shot from each of the 20 or you fire all the ammo of just one and don't use the others.