Using 60Hz clamp meter for measuring current that is not 60Hz
It depends.  There are two ways to read RMS of a specific waveform:


True RMS (expensive)
Simple average of absolute value, then scaled by a constant to read RMS (cheap)


If your meter uses #1, you can measure any waveform that falls entirely within the bandwidth of the meter.  More on that later.

If it uses #2, it'll be wrong for anything other than a sinewave, but if you know the waveform, you might be able to figure out how it's wrong and compensate yourself...maybe.



Concerning waveforms and bandwidth:

There is no such thing as just a meter.  It's a signal processor.  It only happens to have a single slow readout in amplitude.  As a signal processor, it has a range of frequencies (bandwidth) that it will use, and it will ignore everything outside of that range.  This is not a sharp cutoff, but more of a gradual fade-out.  Kinda like turning down both bass and treble and listening to what's left.

Like audio, very few signals are a single frequency, even AC power.  Everything is a mixture of different frequencies.  Some are from the source; some are from the load; all must be captured for an accurate measurement.  The limited bandwidth may cause errors even for 60Hz, depending on the meter and the waveform being measured.

For example, if you're measuring the input current to a linear power supply (AC -> rectifier -> capacitor), then you most definitely DON'T have a sinewave.  It surges at the peak voltage and is "off" otherwise, unless you're looking at a power-factor-corrected (PFC) supply, which has a series of smaller surges to try and approximate a sinewave.  The short time of these surges creates a wide range of frequencies that all have to be considered to get an accurate RMS or even average measurement.  If your meter misses some, you won't get the whole picture.

That being said, if your meter has a wide enough bandwidth to cover complex waveforms at lower frequencies, it will probably handle simpler waveforms at higher frequencies just fine.  Just know what you're dealing with in terms of both signal and instrumentation.