to "whack the clam" is to kick someone, generally a woman, in the pussy, or that thing men shove their dicks into. a "clam" is referring to a "pussy" and a "pussy" refers to a vagina. to whack it is to slap it, grab it, kick it, etc.
"this biddy got into a fight with my girl last night and my girl WHACKED her in the CLAM"-pimpin dude
"i whacked the clam on some bitch last night trying to take my boy"-biddy