Brilliant horizontal experience of an evening? (6)
I believe the answer is:
sunset
'an evening?' is the definition.
(thesaurus)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for sunset that I've seen before include "Period of decline" , "End of the day in terms of light" , "Describes a declining industry or technology" , "This in west" , "Close of day" .)