Wrong ruler is plummeting (7)
I believe the answer is:
sinking
'is plummeting' is the definition.
(thesaurus)
'wrong ruler' is the wordplay.
'wrong' becomes 'sin' (wrong can mean a sin or crime).
'ruler' becomes 'king' (king is a kind of ruler).
'sin'+'king'='SINKING'
(Other definitions for sinking that I've seen before include "Declining" , "failing" , "Going down" , "Falling to the bottom" , "in decline" .)