Now overthrow is gained (3)
I believe the answer is:
won
'is gained' is the definition.
(I've seen this before)
'now overthrow' is the wordplay.
'overthrow' says the letters should be written backwards (I've seen 'overthrown' mean this).
'now' backwards is 'WON'.
(Other definitions for won that I've seen before include "Korean currency, was victorious" , "Succeeded, conquered" , "and 26: Chinese dumpling" , "Gained a victory" , "Came first" .)