Legal action in which rotter falls (7)
I believe the answer is:
cascade
'falls' is the definition.
(synonyms)
'legal action in which rotter' is the wordplay.
'legal action' becomes 'case' (a legal case).
'in which' indicates putting letters inside.
'rotter' becomes 'cad' (a cad is a dishonourable person).
'case' enclosing 'cad' is 'CASCADE'.
(Other definitions for cascade that I've seen before include "Torrent" , "Sudden downpour" , "Small waterfall or series of them" , "Outpouring" , "Rush" .)