x-clipboard-yank insert kill-ring entry instead of primary selection

[mickbert]

Hello.
I am using GNU Emacs 24.3.1 (x86_64-redhat-linux-gnu, GTK+ Version 
3.22.30) of 2018-09-12 on x86-039.build.eng.bos.redhat.com (but the same 
happen with the latest version compiled by me from sources) in an X 
frame

Note that the middle button of mouse, bound to 'mouse-yank-primary, 
works correctly. Instead the command 'x-clipboard-yank (which I bound to 
"<S-ins>", "<S-insert>", "<S-insertchar>" just as experiments) inserts 
the current entry of kill-ring, just the same as 'yank.

Any idea what I can look to find why it does this way, and correct it?
--
Mick