Plenty may be cast (4)
I believe the answer is:
lots
'plenty' is the definition.
(I've seen this before)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for lots that I've seen before include "Large number or amount" , "a crowd?" , "Many found in slot" , "Oodles" , "Tons" .)