The prosecutor's got at in the facts (4)
I believe the answer is:
data
'facts' is the definition.
(I've seen this before)
'the prosecutor's got at' is the wordplay.
'the prosecutor' becomes 'DA' (District Attorney).
'got' means one lot of letters goes inside another.
'da' placed around 'at' is 'DATA'.
'in the' is the link.
(Other definitions for data that I've seen before include "Known facts" , "Listed facts" , "Collection of bits of information" , "Facts and figures as fed to computers (4)" , "What's stored by computers" .)