Use cast during each round (3,4)
I believe the answer is:
eat into
'use' is the definition.
(as in using something up)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for eat into that I've seen before include "start wearing out" , "Corrode (something)" , "run down" , "Consume (resources)" , "Etch" .)