Attorney volunteers facts (4)
I believe the answer is:
data
'facts' is the definition.
(I've seen this in another clue)
'attorney volunteers' is the wordplay.
'attorney' becomes 'DA' (abbreviation for District Attorney).
'volunteers' becomes 'TA' (abbreviation for Territorial Army).
'da'+'ta'='DATA'
(Other definitions for data that I've seen before include "tables, perhaps" , "Series of measurements" , "Organised information" , "Numerical information" , "Listed facts" .)