Prime (6)
(Other definitions for amazon that I've seen before include "Net giant" , "S. America's longest river" , "Female warrior - parrot - South American river" , "Brazilian river" , "World's largest river" .)
'prime' is the definition.
(I know that prime can be written as heyday)
This is the entire clue.
(Other definitions for heyday that I've seen before include "Peak or prime" , "Time of greatest popularity" , "Highest period in someone's career" , "Time of maximum prosperity" , "Boom time" .)
(Other definitions for grade a that I've seen before include "Top quality in alphabetic designation" , "Best quality" , "Top quality or classification" .)