clip layers in spatialite

I'm pretty new to databases and SQL, so sorry for the noise..

I have 2 layers and I want to make a clip of layer 1 with layer 2 in SpatiaLite (I'm using the DB Manager within QGIS).

That's the code I'm using:

create table clipped_lines as
select * from lines, estensione
where st_intersects(lines.GEOMETRY, extension.geom)

The table is created but it is not a geographical object so I cannot add it to the QGIS map canvas.

The problem (I think) is that the geom column is seen as a normal column and not as a GEOMETRY column. I've tried to update the column with:

ALTER TABLE clipped_lines
ALTER COLUMN geom TYPE GEOMETRY(LineString,4326);

but nothing happened.

I thought that the st_intersects function updates automatically the geom column..

Thanks