Father volunteers information (4)
I believe the answer is:
data
'information' is the definition.
(thesaurus)
'father volunteers' is the wordplay.
'father' becomes 'da' (I've seen this in other clues).
'volunteers' becomes 'ta' (abbreviation for Territorial Army).
'da'+'ta'='DATA'
(Other definitions for data that I've seen before include "Collection of items of information" , "Factual information" , "tables, perhaps" , "Collected information" , "Evidence" .)