Corporation supporting US lawyer in fact (5)
I believe the answer is:
datum
'fact' is the definition.
(thesaurus)
'corporation supporting us lawyer' is the wordplay.
'corporation' becomes 'tum' (I've seen this before).
'supporting' means one lot of letters go next to another (in a down clue, some letters go below others).
'us lawyer' becomes 'da' (abbreviation for District Attorney).
'tum' put after 'da' is 'DATUM'.
'in' acts as a link.
(Other definitions for datum that I've seen before include "Little bit of information" , "bit of info" , "Given fact" , "One item of information" , "Fact or stat" .)