Throwing actors in gaol, initially (7)
I believe the answer is:
casting
'throwing' is the definition.
('cast' can be a synonym of 'throw')
'actors in gaol initially' is the wordplay.
'actors' becomes 'cast' (group of actors).
'initially' indicates taking the first letters.
The first letter of 'gaol' is 'g'.
'cast'+'in'+'g'='CASTING'
(Other definitions for casting that I've seen before include "Shaping metal in mould" , "-- Vote; -- couch" , "The process of selecting actors for play" , "Selecting players" , "shell across back of fort" .)