If something..

[caesar]

if {![isop $nick $chan]} { bla bla bla }
I want to make it to see if $nick is not an op in the $chan to continue the proc. Is this correct?

[Mordred]

Yes thats correct. For a further example, maybe check this page in Suninet's guide.

http://www.suninet.nl/tclguide/index.php?chap=10&pg=-1