On some days, volunteers information (4)
I believe the answer is:
data
'information' is the definition.
(data are bits of information)
'on some days volunteers' is the wordplay.
'on' is a charade indicator (letters next to each other).
'some' becomes 'a'.
'days' becomes 'd' (abbreviation for days).
'volunteers' becomes 'TA' (abbreviation for Territorial Army).
'a' put after 'd' is 'da'.
'da'+'ta'='DATA'
(Other definitions for data that I've seen before include "information from bank" , "Evidence" , "Series of measurements" , "Collection of items of information" , "Listed facts" .)