Reddish-brown uranium in a stream (6)
I believe the answer is:
auburn
'reddish-brown' is the definition.
(synonyms)
'uranium in a stream' is the wordplay.
'uranium' becomes 'U' (U is the chemical symbol for uranium).
'in' indicates putting letters inside.
'stream' becomes 'burn' (a burn is a small stream).
'a'+'burn'='aburn'
'u' put inside 'aburn' is 'AUBURN'.
(Other definitions for auburn that I've seen before include "Name of Goldsmith's deserted village (6)" , "Of hair colour, reddish brown (6)" , "Reddish-brown hair colour" , "Golden" , "Goldsmith: 'Sweet . . . . . . loveliest village of the plain'" .)