US prosecutor volunteers information (4)
I believe the answer is:
data
'information' is the definition.
(I've seen this before)
'us prosecutor volunteers' is the wordplay.
'us prosecutor' becomes 'DA' (district attorney).
'volunteers' becomes 'TA' (abbreviation for Territorial Army).
'da'+'ta'='DATA'
(Other definitions for data that I've seen before include "Contents of some banks?" , "Collected information" , "Raw facts" , "Information for/from computer" , "Facts - Star Trek android" .)