You can eat the crackling if you like duck I would. The main point is that you eat ON AVERAGE a normal diet on NFDS. So if out of 5 NFDs per week you have 3 where you eat under your TDEE and 2 where you eat over then that is fine. How much did you eat? 50g of pork crackling is about 275 cals, which is not much more than a mars bar (and far less than the profriteroles I had last week )
If you always ate the crackling before you started 5:2 then of course you can carry on eating it It's the habits on the 2 fasting days that need to change. The key is not to eat the deficit calories from a fast day on a NFD. Been lurking on here since I started 5:2 in January...reached my goal about a month ago almost 2 stone lost so simply...amazing!