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' means one lot of letters goes inside another.
'rotter' becomes 'cad' (a cad is a dishonourable person).
'case' going around 'cad' is 'CASCADE'.
(Other definitions for cascade that I've seen before include "Torrent" , "Sudden downpour" , "Small waterfall or series of them" , "Outpouring" , "Rush" .)