BGMCLEAR is not a command in SmileBasic V1.
If the command has no parameters, it will clear all song data. If it has one parameter, it must be a song number between 128 and 255 inclusive, and that particular song data is erased.
For example, immediately after executing
BGMCLEAR 128, the command
BGMPLAY 128 will not produce any sounds.
No information on SmileBasic V3.