Last but not least (7)
I believe this is a double definition.
'last' is the first definition.
(synonyms)
'not least' is the second definition.
(I've seen this before)
'but' acts as a link.
(Other definitions for extreme that I've seen before include "too much" , "Furthest or highest degree of something" , "Meet Rex (anag.)" , "Most remote or utmost" , "Sharp" .)
(Other definitions for verb that I've seen before include "Maybe protest" , "'In grammar, it denotes action (4)'" , "Part of speech" , "Old-fashioned art, for example" , "Type of word indicating action" .)