Backed commercial corporation in fact (5)
I believe the answer is:
datum
'fact' is the definition.
(thesaurus)
'backed commercial corporation' is the wordplay.
'backed' shows that the letters should be reversed in order.
'commercial' becomes 'ad'.
'corporation' becomes 'tum' (I've seen this before).
'ad' reversed gives 'da'.
'da'+'tum'='DATUM'
'in' acts as a link.
(Other definitions for datum that I've seen before include "Unit of dope" , "Premiss, thing taken for granted" , "Given item" , "Single piece of information" , "Given piece of information" .)