Rich with a hundred, in truth (4)
I believe the answer is:
fact
'in truth' is the definition.
(I've seen this before)
'rich with a hundred' is the wordplay.
'rich' becomes 'fat' ('fat' can be a synonym of 'rich').
'with' indicates putting letters inside.
'a hundred' becomes 'c' (Roman numeral).
'fat' placed around 'c' is 'FACT'.
(Other definitions for fact that I've seen before include "Fait accompli" , "copyright protection agency?" , "Statement of verifiable information" , "matter of record" , "Datum" .)