Something wicked by ruler in decline (7)
I believe the answer is:
sinking
'in decline' is the definition.
(I've seen this before)
'something wicked by ruler' is the wordplay.
'something wicked' becomes 'sin' (I've seen this in other clues).
'by' says to put letters next to each other.
'ruler' becomes 'king' (king is a kind of ruler).
'sin'+'king'='SINKING'
(Other definitions for sinking that I've seen before include "Falling to the bottom" , "in decline" , "failing" , "Going down" , "Declining" .)