Answer:

UMP

Answer	Crossword Clue
UMP	Baseball's ref.
UMP	Fair-or-foul decider
UMP	Ref

UMP	Baseline enforcer
UMP	One who spends the whole game making calls and who might be accused of not watching at all
UMP	One might call you out
UMP	Ref's counterpart
UMP	Diamond arbiter
UMP	Strike caller (abbr.)
UMP	Ballpark figure
UMP	Diamond official

UMP	Masked official
UMP	Diamond authority
UMP	Strike caller
UMP	One making decisions at home?
UMP	Caller at home
UMP	Crouching judge
UMP	Plate manager
UMP	Baseball arbiter

UMP	Certain mask wearer
UMP	One who decides what's fair
UMP	Diamond expert?
UMP	Plate cleaner, at times
UMP	Base decision maker
UMP	Call a strike, say
UMP	Duster at home, sometimes
UMP	Masked one at home

UMP	Official in black
UMP	One wearing black at home
UMP	Diamond caller
UMP	One stationed at a base
UMP	One who wears a mask every fourth game
UMP	"Steee-rike!" caller
UMP	Baseball official
UMP	Call the game

UMP	One crying foul
UMP	Diamond judge?
UMP	One who calls strikes and balls
UMP	Strike zone enforcer
UMP	Little League official