the trim video feature for editing videos on photos app on imac is not working

Is your OS now High Sierra, as you post suggests? Does Photos do this with all videos, or just some of them? Old ones, new ones? Are these .mov or .m4v or what? Can you export the video as an original? Can you re-import it? Does the import behave differently?

To find where the problem lies, I would do the following as first steps:

• Restart the computer (of course)
• Start in Safe Mode (different for different computers) This bypasses certain potentially disruptive processes.

           Start up your Mac in safe mode - Apple Support 

• Use another library if you have one, or make a small test Library with a few pictures in it and see if the same thing happens.
• Make a new user and see if the same problem recurs with it
• Rebuild your Photos Library by closing Photos and re-launching by option-command-clicking the app icon.

One of these steps may fix the problem, but their failure to work is also diagnostic.

If you are really still using macOS 10.13 High Sierra, you have to make the video controls visible to be able to trim the video. View video clips in Photos on Mac – Apple Support (UK)

Trim a video clip

You can trim a video clip so only a certain part of it plays. For example, you can make a clip stop playing at the point where you want it to end, instead of playing the entire clip. Trimming a video clip doesn’t actually delete footage; you can recover any trimmed footage later.

1. Double-click the video clip to open it.
2. Position the pointer over the clip to display the player controls, then click the Action pop-up menu  and choose Trim.
3. Drag the yellow trim handle at either end to shorten the clip, then click Trim.
4. 
5. Tip: To make more precise edits, click and hold a trim handle to see the individual frames of a video. 
6. To cancel the trim, click the Action pop-up menu and choose Reset Trim.

Mac OS Ventura 13.2.1, hovering over the clip does not produce the Action pop-up menu. I have tried this on several videos with the same result, although once I did see the yellow frame with trim handles, fleetingly. Since then I have not seen it again and can't see the Action pop-up menu either.

Possibly this doesn't work only on very short videos?? I tried it on a longer one and while hovering over the clip doesn't work, clicking on one of the < > on either end does, and I could trim it. On two very short videos, it did not work.

You may want to try, if you can trim the two problematic videos in iMOvie or QuickTime Player. Just to see, if there is something wrong with the two videos. I can even trim very short video clips of just two seconds.