Gourd of Californian atomic punch (8)
I believe the answer is:
calabash
'gourd' is the definition.
(calabash is a kind of gourd)
'californian atomic punch' is the wordplay.
'californian' becomes 'cal'.
'atomic' becomes 'a' (eg in A-test, atomic test).
'punch' becomes 'bash'.
'cal'+'a'+'bash'='CALABASH'
'of' acts as a link.
(Other definitions for calabash that I've seen before include "Tree with hard-shelled fruit" , "Large fruit supplier" , "water-container" , "Tropical evergreen tree, might lash a cab, oddly" , "Pot" .)