Assuming it is more important to keep lines that contain something- than it is to delete lines that contain something:

sed '/something/ { /something-/!d; }' file

This finds all lines containing the string something, and deletes all of those lines unless they also contain something-.

Testing:

$ cat file
something something blah blah
something something- blah blah
something
something-

$ sed '/something/ { /something-/!d; }' file
something something- blah blah
something-

Assuming it is more important to keep lines that contain something- than it is to delete lines that contain something:

sed '/something/ { /something-/!d; }' file

This finds all lines containing the string something, and deletes all of those lines unless they also contain something-.

Testing:

$ cat file
something something blah blah
something something- blah blah
somethingelse
something
something-

$ sed '/something/ { /something-/!d; }' file
something something- blah blah
something-