Yes, HAMMER is a Scrabble word!

Letter values in Scrabble US: H 4, A 1, M 3, M 3, E 1, R 1.

Is it a valid word?

HAMMER is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

the part of a gunlock that strikes the percussion cap when the trigger is pulled
a hand tool with a heavy rigid head and a handle; used to deliver an impulsive force by striking
the ossicle attached to the eardrum
a light drumstick with a rounded head that is used to strike such percussion instruments as chimes, kettledrums, marimbas, glockenspiels, etc.
a heavy metal sphere attached to a flexible wire; used in the hammer throw
a striker that is covered in felt and that causes the piano strings to vibrate
a power tool for drilling rocks
the act of pounding (delivering repeated heavy blows)

Also: power hammer.