Using an escaped string (a.k.a. escaped value):
width: ~"calc(100% - 200px)";
Also, in case you need to mix Less math with escaped strings:
width: calc(~"100% - 15rem +" (10px+5px) ~"+ 2em");
width: calc(100% - 15rem + 15px + 2em);
This works as Less concatenates values (the escaped strings and math result) with a space by default.