Long, like ripe bananas (6)
I believe the answer is:
aspire
'long' is the definition.
(thesaurus)
'like ripe bananas' is the wordplay.
'like' becomes 'as'.
'bananas' indicates anagramming the letters (informal term for mad).
'ripe' with letters rearranged gives 'pire'.
'as'+'pire'='ASPIRE'
(Other definitions for aspire that I've seen before include "rise to a great height" , "I spare (anag.)" , "Have ambition (to achieve something)" , "Set your sights on" , "Aim, have an ambitious plan" .)