I'll send a rapper back with the crack of his ass shitty
If he's acting soft and he cowers
He better come cleaner then Jeru jackin' off when he showers

from Eminem (Ft. Eye-Kyu) – 313 Lyrics on Rap Genius

Meaning

Eminem’s meaning here that if any rapper is showing weakness by the littlest bit “If he' acting soft and he cowers”, Em’s going to literally make them shit their pants with his harsh flow.

So he’d better come clean (be straight-up, don’t cower, admit Eminem is the superior emcee). Hence the wordplay with “Come Clean” — achieving orgasm in the shower is “coming” clean, while Jeru the Damaja released the 1994 hip-hop classic “Come Clean.”

Also worth noting the first appearance of Em’s “laugh ad-lib” after a clever line.

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