Huge celebrity revealed overthrown autocrat's age merrily (8)
I believe the answer is:
megastar
'huge celebrity' is the definition.
(I've seen this before)
'revealed overthrown autocrat's age merrily' is the wordplay.
'overthrown' is a reversal indicator.
'revealed' indicates the answer is hidden within the clue (I've seen 'reveals' mean this).
'ratsagem' is hidden within 'autocrats age merrily'.
'ratsagem' backwards is 'MEGASTAR'.
(Other definitions for megastar that I've seen before include "Very famous artist" , "Big personality" , "Huge celebrity" , "famous entertainer" , "A-list celeb" .)