Crossword Puzzle Composure under duress

Clue: Composure under duress

Underwater breathing aids (GILLS with 5 letters)
Understand, as a joke (GET with 3 letters)
Underground bud (BULB with 4 letters)
Underwear with underwire (BRA with 3 letters)
Hint for understanding this puzzle's starred answers (BACKTOSQUAREONE with 15 letters)
Underground party (RAVE with 4 letters)
Uses an Underwood (TYPES with 5 letters)
Bolivian underground? (TINMINE with 7 letters)
Underbosses' bosses (DONS with 4 letters)
Underwriters' tasks (IPOS with 4 letters)