the inserted portion needs be far enough in to be below the bottom of the join of the top tube and the seat tube..
Kalloy as mentioned,are a bargain, Plain sort to mount a saddle clip even more so..
Belt and Braces approach...
unless you read the markings on the botom of the post saying size..
Take the seatpost you have to the bike shop, to make sure the size is proper.
too small damages the frame trying to grip it..


Going upmarket a ways you get a nice stepless adjustable saddle mount mech..