Expression clone is a clone (plasmid in the bacteria, or might be a lambda phage in bacteria) which is designed to produce the protein from the DNA insert. Mammalian genes do not function properly in bacteria, so to get bacterial expression from your mammalian cDNA, you would need to place its coding region (that is no introns) immediately adjacent to bacterial transcription or translation control sequences. That artificial construct (which is "expression clone") will produce the pseudo-mammalian protein if put back into bacteria again. Often, that protein can be recognized by the antibodies raised against the authentic mammalian protein, and vice versa.