“Happily for Jennings, the last of the ‘clubhouse lawyers’ is gone, and now everybody is ‘pullin’ for everybody else.” E.A. Batchelor, The Detroit Free Press, April 12, 1914, p21.
Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
1ST USE. 1929. “He [Del Pratt] was the greatest clubhouse lawyer baseball ever knew” (John Kiernan, The New York Times, June 17; Steve Steinberg).