Bright metallic element, blue (5)
I believe the answer is:
aglow
'bright' is the definition.
('aglow' can be similar in meaning to 'bright')
'metallic element blue' is the wordplay.
'metallic element' becomes 'ag' (Ag is an example**).
'blue' becomes 'low' (I've seen this in other clues).
'ag'+'low'='AGLOW'
(Other definitions for aglow that I've seen before include "Illuminated" , "Softly bright or radiant" , "Smouldering" , "Red-hot" , "emitting a steady light" .)