Green’s Dictionary of Slang

1. (US) a fight between travelling carnival or circus people and local townspeople.

1891		W. De Vere ‘Hey Rube’ Tramp Poems 88: When e’er a circus comes your way, And you are spilein’ for ‘clim,’ Be sure they haven’t learned to sing ‘Hey Rube’.
1905		Little Falls Herald (MN) 31 Mar. 3/3: How to Operate the Shell Game with Profit [...] Hunch him [i.e. the loser] over to the juice joint and throw the bull con, but lead him off the lot before he gets hep and makes a roar for his wad, or starts a clemm.
1924		(ref. to c. 1890) F. Isman Weber and Fields 100: Clems, as such shindigs were known in circus argot, and their battle cry, ‘Hey Rube!’.
1931		Irwin Amer. Tramp and Und. Sl. 52: Clem. – A general fight or riot. Originated in the old days of horse-drawn circuses when the townspeople or ‘towners’ and the show folk were usually at odds.
1935		A.J. Pollock Und. Speaks 22/2: Clem, a fight.
1949		F. Brown Dead Ringer 106: All I had on was a thin rayon dressing gown, and nothing under it but a g-string. I wouldn’t tangle with a crowd of rubes, dressed like that. It’d start a clem, with me in the middle.
1951		D.P. Mannix Sword-Swallower 75: During the clem the townies had cut the cable running to the juice truck.
2012		W. Keyser ‘Carny Lingo’ in http://goodmagic.com 🌐 Clem [...] a fight between a townie and carnies.

2. (Irish) a second-rate thing.

1969

P. Boyle All Looks Yellow to the Jaundiced Eye 105: Sally was to tell her folks that I was anxious to punch in a bit of practice on a first-class instrument, Aunt Mary’s piano being a clem.

3. a visitor to the carnival, thence a gullible individual.

2012

W. Keyser ‘Carny Lingo’ in http://goodmagic.com 🌐 Clem — Another term for ‘mark,’ particularly a gullible rural local.