Start of spectrum initially rearranged for rave-up (4)
I believe the answer is:
orgy
'start' is the definition.
The definition and answer can be both acts as well as being singular nouns.
Maybe they are linked in a way I don't understand?
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for orgy that I've seen before include "Binge" , "Unrestrained revel" , "Riotous revel" , "Wild activity" , "overindulgence" .)