Daughter joins a corporation - that's a fact (5)
I believe the answer is:
datum
'that's a fact' is the definition.
(singular form of 'data')
'daughter joins a corporation' is the wordplay.
'daughter' becomes 'd' (genealogical abbreviation).
'joins' means one lot of letters go next to another.
'corporation' becomes 'tum' (I've seen this before).
'd'+'a'+'tum'='DATUM'
(Other definitions for datum that I've seen before include "Basic fact" , "Something given" , "Minimal information" , "Little bit of information" , "Given fact" .)