Fact spinning commercial corporation (5)
I believe the answer is:
datum
'fact' is the definition.
(singular form of 'data')
'spinning commercial corporation' is the wordplay.
'spinning' says the letters should be written backwards.
'commercial' becomes 'ad' (abbreviation for advertisement).
'corporation' becomes 'tum' ('corporation' is a humorous term for stomach fat).
'ad' written backwards gives 'da'.
'da'+'tum'='DATUM'
(Other definitions for datum that I've seen before include "Basic fact" , "Given piece of information" , "given to the Romans" , "Given fact" , "Single informative fact" .)