Using ClippingSticker app for macOS (OS X) it should be pretty easy to export pictures from .clipping files and save them in any of the supported graphics formats, including PNG graphics (Portable Network Graphics). This seems like one of the most convenient ways how to achieve clipping to png conversion.
Convert Mac Text clipping file (prior Mac OS 9) to Portable Network Graphic.
We found one software available in the database for this conversion.
Apple macOS / Mac OS X software - convert clipping to png on OS X
A program for exporting clipping images
Convert from clipping file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.