Attorney to volunteer information (4)
I believe the answer is:
data
'information' is the definition.
(data are bits of information)
'attorney to volunteer' is the wordplay.
'attorney' becomes 'DA' (district attorney).
'to volunteer' becomes 'TA' (member of the Territorial Army, volunteer regiment).
'da'+'ta'='DATA'
(Other definitions for data that I've seen before include "Known facts" , "Contents of some banks?" , "Statistical information" , "Pieces of information" , "Information for/from computer" .)