Dark region of Museum Brandhorst (5)
I believe the answer is:
umbra
'dark region' is the definition.
(I've seen this before)
'of museum brandhorst' is the wordplay.
'of' indicates the answer is hidden within the clue.
'UMBRA' is hidden within 'museum brandhorst'.
(Other definitions for umbra that I've seen before include "Full shadow" , "Black spot" , "Area of total shadow" , "Burma (anag.)" , "Shade, or ghost" .)