Pardon the question but how does a Photoshop "clipping mask" differ from a regular layer mask?
I ask because, in the GIMP, a masked layer is editable seemingly just as David requires ... :
... without the need for an additional layer or additional mask?
Not saying that the GIMP is better or worse - just trying to follow the PS terminology ...
FWIW, original and final result of the above can be viewed here:
https://www.dpreview.com/forums/post/63971115
As can be seen, the shadowy area got lightened quite considerably.