Case of Mauritius walnuts overturned (7)
I believe the answer is:
lawsuit
'case' is the definition.
(I know that case can be written as lawsuit)
'of mauritius walnuts overturned' is the wordplay.
'of' says the answer is hidden in the clue.
'overturned' shows that the letters should be reversed in order.
'tiuswal' can be found hidden inside 'mauritius walnuts'.
'tiuswal' back-to-front is 'LAWSUIT'.
(Other definitions for lawsuit that I've seen before include "Process of bringing dispute or claim before the courts" , "Dispute brought to court" , "Claim brought to court" , "Court case" , "The process of bringing action in legal court" .)