If I get enough marks in clat2021 to get admission in the NLSIU Bangalore but my first preference is the RMNLU Lucknow can I get admission in the NSLIU (second preference)?

Since your first preference was RMNLU, and if your scores match up to the criteria followed by the institute, then it is likely that you will secure a seat at RML. The seats at NLSIU also partially are considered important as well. 

The counseling (also known as the seat allotment process) is patterned in a way that students secure seats as per their preferred choice. NLSIU is a highly sought-after institute and it is the first preference for many students. 

Now, there are around 80 seats available at NLSIU including the seats for reserve category students. So, if your rank is within AIR 80~85, there is a good chance that you will secure a seat at NLSIU. 

However, if you have an AIR of around 65 and keep NLSIU as your 2nd preference then you will certainly get RMNLU and will be out of the race for NLSIU.