In action, organ proved successful (9)
I believe the answer is:
delivered
'proved successful' is the definition.
(as in to deliver on a promise or goal)
'in action organ' is the wordplay.
'in' indicates putting letters inside.
'action' becomes 'deed' (deed is a kind of action).
'organ' becomes 'liver' (liver is a kind of organ).
'deed' placed around 'liver' is 'DELIVERED'.
(Other definitions for delivered that I've seen before include "Brought to a destination" , "Saved" , "Produced new issue" , "sent to the house" , "Handed over" .)