got a small cut (about 7mm long) in the rubber on the side of my tubeless tyre (specialized captain). I can see some of the tyre threads through it but they tyre isnt losing air.

Can i fix it / reinforce it from the outside with some glue and a patch? Dont want to take off if possible as its full of gloop.

Or is it likely to fail horribly on the next rocky descent so i should just get a new one?

Just fit an external patch – should work fine

deflate it a bit
put some superglue in the cut
squeeze the cut together while it dries
reinflate

this has worked for me on a number of occasions.