Witness affair, furious (8)
I believe the answer is:
seething
'furious' is the definition.
(I've seen this before)
'witness affair' is the wordplay.
'witness' becomes 'see' (I've seen this before).
'affair' becomes 'thing' ('thing' can be a synonym of 'affair').
'see'+'thing'='SEETHING'
(Other definitions for seething that I've seen before include "Bubbling, perhaps with rage" , "Observe object - bubbling with rage" , "very hot" , "Boiling" , "making one very angry" .)