Answer:
What can be underwater, under fire, on fire and taking water all at the same time?
I hide but my head is outside.
Gets rid of bad ones, short and tall. Tightens when used, one size fits all.
My first is in blood and also in battle. My second is in acorn, oak, and apple. My third and fourth are both the same. In the center of sorrow and twice in refrain. My fifth starts eternity ending here. My last is the first of last, Oh Dear.
All about, but cannot be seen, Can be captured, cannot be held, No throat, but can be heard. Who am I?
I'm the source of all emotion but I'm caged in a white prison.