if(IsPlayerAdmin(playerid)) // if player is rcon admin
{
    // proceed to your code (can use setclans command)
}else
{
    // when player is not rcon admin (can't use setclans command)
}

dcmd_setclans(playerid, params[])
{
   If(IsPlayerAdmin(playerid)
   {
      // do the set clan
    }
    Else
    {
       // dont do it
    }
}

dcmd_setclans(playerid,params[])
{
    #pragma unused params
    if(!IsPlayerAdmin(playerid)) // If the player is not a rcon admin, it shows the above message to the player.
    {
        return SendClientMessage(playerid, -1, "You need to be a rcon admin to use this command!");
        return 1;
    }
    else // If he is a rcon admin, it completes the command
    {
        format(pS[String], 128, "");
        for(new p = 0;p <10; p++)
        {
            format(pS[String], 128, "%s%s\n",pS[String],ClanInfo[p][fName]);
        }
        return ShowPlayerDialog(playerid,ClanDialog+15, DIALOG_STYLE_LIST,"Clan List", pS[String], "Set", "Close");
    }
    return 1;
}