What's the closest replacement for clam juice?
Cook's Thesaurus recommends fish stock or 50:50 chicken stock &amp; water.

As I don't tend to make fish stock, I'd probably either use shrimp stock (take the shells and heads, simmer in water for about an hour), or crab stock (just use the shells, and maybe the smaller legs if you didn't extract the meat from them)

If you can find it, some stores will carry 'seafood bullion' or 'shrimp bullion'.  As they tend to be salty, I'd probably only use them at half strength, like the recommendation for substituting chicken stock.

In a real pinch, you might be able to use water + fish sauce, but I have no idea what the proportions would be.  (I'd probably compare sodium levels, and try to match that.  So 1400mg/TB vs. 120-280mg/2oz ... so averages 800mg/cup, so about 1.5tsp fish sauce + water to replace a cup of clam juice).