Where are you going to plant this 'tough yellow evergreen'? Sun, shade, soil conditions, how big do you want it to grow? More info needed please.

Choysia "Sundance" springs to mind, and some junipers - e.g. Juniperus Chinensis 'Aurea', but, as MG says, "Horses for Courses" - conditions and a list of requirements would be useful.

My old standby Euonymus Fortunii Emerald 'n gold id you want something that doesn't get very tall. (Except that it will send long shoots up a wall if planted next to it)

That's what I would suggest, too, Steragram.

And me!

If you want something bigger, Elaeagnus pungens 'Frederici' has pale yellow leaves with a green rim. E. x ebbengei 'Gilt Edge' with its yellow edge to the leaves is strikingly yellow in winter, tough as old boots, doesn't mind a fair bit of shade either. Otherwise, Euonymus japonica 'aureopictus' has good yellow variegation.

Yes, eleagnus is a great idea