I’ve attempted to add extra verbose comments so that no further explanation is needed:

#include <chrono>
#include <iostream>
#include <sstream>

int
main()
{
    using namespace std;
    using namespace chrono;

    // Inputs
    
    // duration as string
    string tod_s = "13:00:00";
    // Some utc_time from UTC string
    utc_seconds tp_utc = floor<seconds>(clock_cast<utc_clock>(system_clock::now()) + 6h);
    // Some time zone
    string tz_s = "America/New_York";

    // Compare today's midnight in time zone + duration against utc_time

    // Convert tod_s to duration
    istringstream in{tod_s};
    in.exceptions(ios::failbit);
    seconds tod;
    in >> parse("%T", tod);

    // Get pointer to time_zone
    auto tz = locate_zone(tz_s);

    // Get local midnight of today
    auto local_mid = floor<days>(tz->to_local(system_clock::now()));
    // Add duration to get local time point
    auto tp_local = local_mid + tod;

    // Convert local time to sys_time, and then to utc_time
    auto tp = clock_cast<utc_clock>(tz->to_sys(tp_local));

    // Compare tp and tp_utc
    bool b = tp > tp_utc;
}

A comment though: You might want to use sys_time in place of utc_time. sys_time is what computers normally traffic in. utc_time adds leap seconds to it. You really only want to do this when you know that you need the leap seconds in. This is typically for spacecraft or astronomical applications.

To change to using sys_time, simply remove the clock_cast to utc_time at the last step.