Greetings Sunny, 

If you are eligible for defence quota and there is only 1 seat then there will be competition as the candidate with better number and rank will get the chance irrespective of the quota and even if there is competition in quota itself also then the best will be selected in terms of rank and number.But there is nothing to worry about contact your college they might give you the best suggestions. 

Good Luck!!

Regards Ankit!!