Method of choice covering specific period (9)
I believe the answer is:
operation
'method of choice covering specific period' is the definition.
The answer and definition can be both natural processes as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
'choice covering specific period' is the wordplay.
'choice' becomes 'option' (synonyms).
'covering' indicates putting letters inside.
'specific period' becomes 'era' (era is a kind of period. I am not sure about the 'specific' bit.).
'option' placed around 'era' is 'OPERATION'.
Can you help me to learn more?
(Other definitions for operation that I've seen before include "financial transaction" , "Act of surgery" , "working" , "battle" , "management" .)