Mineral in bananas (9)
I believe the answer is:
potassium
'mineral in bananas' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for potassium that I've seen before include "Soft metallic element" , "Silvery metallic element" , "Reactive metal element" , "Element K" , "Soft metal, symbol K" .)