Shining helmet worn by copper travelling west (5)
I believe the answer is:
lucid
'shining' is the definition.
(I've seen this before)
'helmet worn by copper travelling west' is the wordplay.
'helmet' becomes 'lid' (slang for crash helmet).
'worn by' indicates putting letters inside (I've seen this in other clues).
'copper' becomes 'cu' (chemical symbol).
'travelling west' shows that the letters should be reversed in order.
'cu' in reverse letter order is 'uc'.
'lid' placed around 'uc' is 'LUCID'.
(Other definitions for lucid that I've seen before include "Clearly presented and easily understood" , "Clearly expressed and altogether rational" , "Readily understood" , "Cogent" , "Compos mentis" .)