Dull, going over sierra in damp part of US (7)
I believe the answer is:
midwest
'part of us' is the definition.
('Midwest' is part of 'us')
'dull going over sierra in damp' is the wordplay.
'dull' becomes 'dim' (synonyms).
'going over' shows that the letters should be reversed in order.
'sierra' becomes 'S' (phonetic alphabet: alpha, bravo, charlie etc.).
'in' indicates putting letters inside.
'damp' becomes 'wet' (similar in meaning).
'dim' reversed gives 'mid'.
's' going into 'wet' is 'west'.
'mid'+'west'='MIDWEST'
(Other definitions for midwest that I've seen before include "Area from Ohio to the Rockies" , "somewhere in America" , "Mist dew (anag.)" , "US states including Illinois" , "Region from Ohio to the Rockies" .)