Playwright once releasing book to be grabbed by males ready for casting? (6)
I believe the answer is:
molten
'ready for casting?' is the definition.
'playwright once releasing book to be grabbed by males' is the wordplay.
'playwright once' becomes 'bolt' (Robert Bolt).
'releasing' indicates named letters should be taken away.
'book' becomes 'b'.
'to be grabbed by' means one lot of letters goes inside another.
'males' becomes 'men' (man is a kind of male).
'bolt' with 'b' removed is 'olt'.
'olt' inserted within 'men' is 'MOLTEN'.
(Other definitions for molten that I've seen before include "State of moving lava" , "Liquid (of metal)" , "hot but ready to pour" , "Liquefied by heat" , "Reduced to liquid form by heating" .)