Precious metal base is shining (5)
I believe the answer is:
aglow
'is shining' is the definition.
(I've seen this in another clue)
'precious metal base' is the wordplay.
'precious metal' becomes 'ag' (Ag is the chemical symbol for silver).
'base' becomes 'low' (synonyms).
'ag'+'low'='AGLOW'
(Other definitions for aglow that I've seen before include "Giving out a steady light" , "Incandescent" , "Flushed and rosy" , "Lit up" , "to be exhibiting radiance" .)