Green’s Dictionary of Slang

(US Und.) to explain; to find out information.

1925		D. Hammett ‘Tom, Dick, or Harry’ in Nightmare Town (2001) 245: We’ll clean up all we can on this kid [...] and then see what further damage we can do.
1992		Bentley & Corbett Prison Sl. 14: Clean It Up To explain, clarify or take care of something.