Criminal came up with lawyer initially entering a confession (3,5)
I believe the answer is:
mea culpa
'confession' is the definition.
(thesaurus)
'criminal came up with lawyer initially entering a' is the wordplay.
'criminal' indicates anagramming the letters.
'with' says to put letters next to each other.
'initially' says to take the initial letters.
'entering' means one lot of letters goes inside another.
The initial letter of 'lawyer' is 'l'.
'came'+'up'='cameup'
'cameup' anagrammed gives 'meacup'.
'meacup' placed around 'l' is 'meaculp'.
'meaculp'+'a'='MEA CULPA'
(Other definitions for mea culpa that I've seen before include "Acknowledgement of one's own fault (Latin)" , "Admission of error (Latin)" , "Old Roman admission of guilt" , "Sorry!" , "Acknowledgement of one's guilt (Latin)" .)