Instant flour is a granular flour that dissolves quickly to thicken gravies, sauces, custards and other dishes. Wondra is a brand of instant flour.
Instant flour is low protein, finely ground flour that has been treated so that it will dissolve instantly in water and not require the same long cooking process as non-instant flour to dissolve in a liquid and thicken it. The process is called pregelatinization, and it involve heat a starch (flour) with very hot water and/or steam, then drying it out, so that it has essentially been cooked already. Because of this, instant flour also very unlikely to form lumps when mixed with a water or other solution. Wondra is a brand of instant flour and has some malted barley flour mixed into it, which acts as a dough conditioner in many breads.
While all purpose flour, pastry flour and cake flour can all be substituted for each other reasonably well when you make a few adjustments to the amounts you need, you cannot really work out a similar substitution for instant flour. Cake flour, with its low protein content, will be the closest you can get, but unless only a very small amount is called for in your recipe, it is worth either (a) trying to find some Wondra or (b) trying to find a different recipe.