Underhand

Underinclined

(think I made that up!)

Well, language is ever changing and why should we not make our contribution?!

Under er, er, ... under-jammed (my newly minted term for a tart with too much pastry and insufficient jam )

We've all eaten one of those in our time, Andere, so I agree that it should be a word.

I don't think the following would appear in any dictionary any time soon:
K for underkip (for when you wake up tired and need more sleep)

There's a lot of that about at present. Oh, for a decent night's sleep ...

Underlay

Undermanned

Underneath

Underoptimistic

Underpaid

Underqualified

Under-resourced

undersold

Undertaken

Underused

Undervalued