Shimmering river rising over a track (7)
I believe the answer is:
opaline
'shimmering' is the definition.
Both the definition and answer are adjectives. Maybe you can see an association between them that I don't see?
'river rising over a track' is the wordplay.
'river' becomes 'Po' (Italian river).
'rising' shows that the letters should be reversed in order (in a down clue, letters go up).
'over' means one lot of letters go next to another (in a down clue, some letters go over others).
'track' becomes 'line' (line is a kind of track).
'po' backwards is 'op'.
'op'+'a'+'line'='OPALINE'
Can you help me to learn more?
(Other definitions for opaline that I've seen before include "Photographic plate" , "Like some gemstones" , "of stones" , "Translucent glass" , "Milk glass" .)