Promotional material backing corporation statistic (5)
I believe the answer is:
datum
'statistic' is the definition.
(I've seen this in another clue)
'promotional material backing corporation' is the wordplay.
'promotional material' becomes 'ad' (ad is a kind of promotional material).
'backing' says the letters should be written in reverse.
'corporation' becomes 'tum' ('corporation' can humorously mean belly fat).
'ad' backwards is 'da'.
'da'+'tum'='DATUM'
(Other definitions for datum that I've seen before include "given to the Romans" , "Given point or position" , "Basic fact" , "Given fact" , "Little bit of information" .)