As a diet of bananas, with a spice (10)
I believe the answer is:
asafoetida
'spice' is the definition.
(I know that asafoetida is a type of spice)
'as a diet of bananas with a' is the wordplay.
'bananas with' indicates anagramming the letters (informal term for mad).
'diet'+'of'='dietof'
'dietof' mixed up with 'a' can make 'foetida'.
'as'+'a'+'foetida'='ASAFOETIDA'
(Other definitions for asafoetida that I've seen before include "Fragrant plant" , "Smelly plant" , "Indian ingredient" , "it creates a stink" , "Gum" .)