Facts somewhat overturned (4)
I believe the answer is:
data
'facts' is the definition.
(I've seen this before)
'somewhat overturned' is the wordplay.
'somewhat' becomes 'a tad' ('a tad' can mean 'somewhat' or 'slightly').
'overturned' is a reversal indicator.
'atad' written backwards gives 'DATA'.
(Other definitions for data that I've seen before include "Collection of facts, information" , "What's stored by computers" , "Collected information" , "computer input?" , "Facts and statistics collected together" .)