I have failed in physics but passed in chemistry & biology. Total gives me more than 50%. Can I get grace in physics and appear for neet.

Sorry to hear that. But NTA is the exam conducting authority for NEET. And your board exams are conducted by the respective Board authorities. So NTA cannot add grace marks to your subject. Or else you can apply for re-evaluation. They can see if there is a way but that also is not a guarantee. For NEET, you should complete and pass your 12th in all the subjects with 50% and have the certificates by the time of counselling. For this you have to check with your college as what else can be done...