Cummings primarily a toxic element, not primarily a brainbox (7)
I believe the answer is:
cranium
'a brainbox' is the definition.
(I've seen this before)
'cummings primarily a toxic element not primarily' is the wordplay.
'primarily' suggests taking the first letters.
'a toxic element' becomes 'uranium'.
'not primarily' means to remove the first letter.
The initial letter of 'cummings' is 'c'.
'uranium' with its first letter taken away is 'ranium'.
'c'+'ranium'='CRANIUM'
(Other definitions for cranium that I've seen before include "Part of the skull" , "Skull, bones enclosing brain" , "hard case" , "The skull, major head bone" , "Skull that encloses the brain" .)