That which disproves half of forthcoming genuine cases (7)
I believe the answer is:
refutal
'that which disproves' is the definition.
(I've seen this before)
'half of forthcoming genuine cases' is the wordplay.
'half of' means to take half the letters.
'forthcoming' becomes 'future' (similar in meaning).
'genuine' becomes 'real' (associated in meaning).
'cases' indicates putting letters inside.
'future' halved is 'fut'.
'fut' going within 'real' is 'REFUTAL'.
(Other definitions for refutal that I've seen before include "That which disproves" , "denial" .)