Finally fined a corporation, in fact (5)
I believe the answer is:
datum
'fact' is the definition.
(singular form of 'data')
'finally fined a corporation' is the wordplay.
'finally' says to take the final letters.
'corporation' becomes 'tum' ('corporation' is a humorous term for stomach fat).
The final letter of 'fined' is 'd'.
'd'+'a'+'tum'='DATUM'
'in' acts as a link.
(Other definitions for datum that I've seen before include "Something given" , "Given fact" , "Single informative fact" , "Basic fact" , "Single piece of information" .)