Under control, remove phosphorous and boron from copper zinc alloy (2,5)
I believe the answer is:
in check
'under control' is the definition.
(synonyms)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for in check that I've seen before include "Restrained" , "Under control" , "Attacked" , "State of chess king under direct attack" .)