Bananas are able to develop (9)
I believe the answer is:
elaborate
'develop' is the definition.
(I know that develop is a more specific form of the action elaborate)
'bananas are able to' is the wordplay.
'bananas' indicates an anagram (informal term for mad).
'are'+'able'+'to'='areableto'
'areableto' is an anagram of 'ELABORATE'.
(Other definitions for elaborate that I've seen before include "Give more details" , "Complicated - complex" , "with added detail" , "Complicated or ornate" , "Describe in detail, being complicated" .)