tonynannini - The instruction applied to the 'hidden word' is similar to what you've done to get the thematic entries, but operating in the opposite direction (as you would expect, because this time you're looking to get the initial answer - the 'thematic entry' is already in the grid).
Once you've got the 'instruction' it narrows down the options for the 'hidden word', because it shares a cell with the instruction (and you know how many cells are involved in total).
I hope this is a bit more helpful (sorry it's so long-winded!).