Any good substitutes for tamarind in Beef Rendang?
Tamarind gives a sour and slightly molasses note to food, including rendang. To replicate it, you thus need something sour, like lemon juice, and a dark sugar flavour, like black treacle, molasses, or dark brown sugar.

For your recipe I would add 2 tablespoons of lemon juice and 1 tablespoon of dark brown sugar.