You used notation:
$f[x,f[x]]$ - does this make any sense? Your first f depends on 2 arguments but second f depends just on 1 argument? Why are you not posting actual functions and code that you are using? It gives me impression that you did not really try to actually code it or did not describe actual issue, because there are no problems in plotting nested functions:
Plot[JacobiSN[JacobiSN[x, 1/3], 1/3], {x, -10, 10}]
