About time Maud reformed; that's a fact (5)
I believe the answer is:
datum
'a fact' is the definition.
(thesaurus)
'about time maud reformed' is the wordplay.
'about' is an insertion indicator.
'time' becomes 't'.
'reformed' indicates an anagram.
'maud' anagrammed gives 'daum'.
't' going inside 'daum' is 'DATUM'.
'that's' acts as a link.
(Other definitions for datum that I've seen before include "Little bit of information" , "bit of info" , "Premiss, thing taken for granted" , "Bit of dope" , "Single informative fact" .)