Answer: jumpin

AnswerCrossword Clue
JUMPINFats Waller's "The Joint is .."
JUMPINGet wet fast
JUMPINEnter enthusiastically
JUMPINEnter without much planning
JUMPINGet involved eagerly
JUMPINLike Jack Flash
JUMPINInterrupt
JUMPINLike a hot club, informally
JUMPIN"Be my guest!"