word - A Riddle Like a Phoenix

Set right within a wood, you see
An army group with energy.
Addendum in the heart may be
Guffawing inappropriately.

Behold! O sad remains the key
That holds the ace front backwardly

The answer is a single English word.

Answer

The answer is

CORPSE.

Right within a wood you see

Right (R) within a wood (COPSE).

An army group with energy.

An army group (CORPS) with energy (E).

Addendum in the heart may be

Addendum (PS) in the heart (CORE) - thanks to @Deusovi for catching this.

Guffawing inappropriately.

Corpsing, for an actor, means to guffaw inappropriately.

Behold! O sad remains the key
That holds the ace front backwardly

Pretty sure this is another piece of wordplay - perhaps something to do with the letter O, or "ace front" meaning A? Or as Volatility suggests, behold -> see -> C, then O, then the rest somehow cluing RPSE?

OP edit: Putting together the last pieces. On the last two lines I attempted (perhaps too successfully) to misdirect the user with punctuation and enjambment (line breaks). These are actually two clues with the split between "remains" and "the key". The first clue is a definition. It becomes "Behold: o sad remains!" or "Lookit the corpse!" The second clue is "The key that holds the ace front backwardly." This refers to the escape key, ESC, holding PRO, a synonym for a skilled person or ace. This gives "ES(PRO)C". Taken "front backwardly": "C(ORP)SE". The title is a reference to the way a phoenix rises from the ashes of its former self; that is, from a corpse. My private joke is that I made this puzzle as an indirect hint for this puzzle, making the title doubly apt. And, yes, I do realize this makes me an insufferable meta-geek.