Cad’s cue to involve defendant (7)
I believe the answer is:
accused
'defendant' is the definition.
(synonyms)
'cad's cue to involve' is the wordplay.
'to involve' indicates anagramming the letters.
'cads'+'cue'='cadscue'
'cadscue' is an anagram of 'ACCUSED'.
(Other definitions for accused that I've seen before include "Levelled a charge against" , "Defendant appearing on a criminal charge" , "Indicted" , "Incriminated" , "One tried" .)