Sue cereal company's head and be ruined in famous trial (5,7)
I believe the answer is:
cause celebre
'famous trial' is the definition.
'sue cereal company's head and be ruined' is the wordplay.
'head' suggests taking the first letters.
'and' is a charade indicator (letters next to each other).
'ruined' is an anagram indicator.
The first letter of 'companys' is 'c'.
'sue'+'cereal'+'c'+'be'='suecerealcbe'
'suecerealcbe' is an anagram of 'CAUSE CELEBRE'.
'in' is the link.
(Other definitions for cause celebre that I've seen before include "Famous controversy" , "Incident that attracts great public attention" , "Controversial case" , "Notorious lawsuit, for example" , "Incident attracting serious controversy" .)