American lawyer close to overturning facts (4)
I believe the answer is:
data
'facts' is the definition.
(data are bits of information)
'american lawyer close to overturning' is the wordplay.
'american lawyer' becomes 'DA' (abbreviation for District Attorney).
'close to' becomes 'at' (close to a location).
'overturning' shows that the letters should be reversed in order.
'at' backwards is 'ta'.
'da'+'ta'='DATA'
(Other definitions for data that I've seen before include "Known facts" , "Statistical information" , "computer input?" , "Collection of facts, information" , "Numerical information" .)