Crossword puzzle clue "joke inside four of the across answers"

Joke inside four of the Across answers

Letter appearing only in down answers; its opposite appears only in across answers

Org. "formed" inside the four longest answers

HBO's "Inside the __" (and hidden theme in this puzzle's four longest answers)

Service found in the four longest Across answers herein

Fire proof added to the four longest Across answers

Creature wrapped around the four longest Across answers

Apt word to substitute for each of four black squares to make sense of the across answers on either side of them