Bright copper keeping head down during 40 day trial period (6)
I believe the answer is:
lucent
'bright' is the definition.
('lucent' can be similar in meaning to 'bright')
'copper keeping head down during 40 day trial period' is the wordplay.
'copper' becomes 'cu' (chemical symbol).
'keeping head down' is a reversal indicator.
'during' is an insertion indicator.
'40 day trial period' becomes 'lent'.
'cu' written backwards gives 'uc'.
'uc' inserted inside 'lent' is 'LUCENT'.
(Other definitions for lucent that I've seen before include "Softly bright or radiant" , "Brilliant" , "Shining" , "'Lambent, luminous (6)'" .)