Cast starting tragedy fail to remember something (6)
I believe the answer is:
forget
'fail to remember something' is the definition.
(forgetting is the opposite of remembering)
'cast starting tragedy' is the wordplay.
'cast' becomes 'forge' (as in casting or forging from metal).
'starting tragedy' becomes 't' (I can't justify this - if you can you should give a lot more credence to this answer).
'forge'+'t'='FORGET'
(Other definitions for forget that I've seen before include "Be unable to remember" , "Stop remembering" , "Leave behind unintentionally" , "Overlook" , "Fail to remember" .)