Information poster put up by corporation (5)
I believe the answer is:
datum
'information' is the definition.
(singular form of 'data')
'poster put up by corporation' is the wordplay.
'poster' becomes 'ad' (short for advert).
'put up' says the letters should be written in reverse.
'by' is a charade indicator (letters next to each other).
'corporation' becomes 'tum' ('corporation' can humorously mean belly fat).
'ad' in reverse letter order is 'da'.
'da'+'tum'='DATUM'
(Other definitions for datum that I've seen before include "given to the Romans" , "Minimal information" , "Something given" , "Little bit of information" , "Fixed starting point for measurements" .)