Catalysis of the reaction: xanthan = oligosaccharide with 4-deoxy-alpha-L-threo-hex-4-enuronosyl end + pyruvylate mannose. This reaction is the eliminative cleavage of the terminal beta-D-mannosyl-beta-D-1,4-glucuronosyl linkage of the side-chain of the polysaccharide xanthan, leaving a 4-deoxy-alpha-L-threo-hex-4-enuronosyl group at the terminus of the side-chain.