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' shows that the letters should be reversed in order.
'commercial' becomes 'ad' (abbreviation for advertisement).
'corporation' becomes 'tum' ('corporation' is a humorous term for stomach fat).
'ad' in reverse letter order is '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" .)