Answer: playball

AnswerCrossword Clue
PLAYBALL

Cooperate (with)

PLAYBALL

Ump's cry

PLAYBALL

1 Down's directive.

PLAYBALL
Co-operate.
PLAYBALL
Certain opening words
PLAYBALLAn opening line
PLAYBALLDirective
PLAYBALLUmpire's shout
PLAYBALLBook by John Feinstein