A cam is an eccentric - Bradford's has it & Collins gives -
a moving piece of machinery, as a wheel or projection on a wheel, that gives an eccentric rotation or a reciprocating motion to another wheel, a roller, a shaft, etc., or that receives such motion from it
-------------------------------------------------------------
Quietly getting shot of horse in one (in some way)?
Photo finish - 'p(iano; quietly)' + anag 'shot of h(orse) in i (one)'