Shadow some dumb rating (5)
I believe the answer is:
umbra
'shadow' is the definition.
(umbra is a kind of shadow)
'some dumb rating' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'UMBRA' is hidden within 'dumb rating'.
(Other definitions for umbra that I've seen before include "Dark shadow cast by earth in eclipse" , "Region of shadow" , "Full shadow" , "Shade, or ghost" , "shady region" .)