Side with marks, ripe bananas showing surface (9)
I believe the answer is:
epidermis
'surface' is the definition.
(thesaurus)
'side with marks ripe bananas' is the wordplay.
'with' means one lot of letters go next to another.
'marks' becomes 'm' (former German currency).
'bananas' indicates an anagram (informal term for mad).
'm'+'ripe'='mripe'
'side' put after 'mripe' is 'mripeside'.
'mripeside' anagrammed gives 'EPIDERMIS'.
'showing' acts as a link.
(Other definitions for epidermis that I've seen before include "Surface of the skin" , "Outer layer of skin" , "Protective covering" , "Outermost layer" .)