Transparence in Lua?

How can I make a transparance effect in Lua for PSP?
I don't understand the syntaxe of this fonction :
screen:clear([transparant-black])
or
screen:fillRect([transparant-black])

For example: I want make a mouse, because my PNG file is square and shape's mouse is arrow. And I don't want print the "corner".
How is the right syntax ?
Thank you.

I reach to make my cursor program using a picture already transparant.
But How can I make a transparent picture effect ?