Cast, we hear, is finished (7)
I believe the answer is:
through
'is finished' is the definition.
('through' can be similar in meaning to 'finished')
'cast we hear' is the wordplay.
'cast' becomes 'threw' (I've seen this before).
'we hear' shows a homophone (sound like).
'threw' sounds like 'THROUGH'.
(Other definitions for through that I've seen before include "clear" , "made it to the next stage" , "Per" , "By means of, via" , "having arrived at next stage" .)