Mystery writer covering bananas in fragrant ointment (6)
I believe the answer is:
pomade
'fragrant ointment' is the definition.
(ointment used to style hair)
'mystery writer covering bananas' is the wordplay.
'mystery writer' becomes 'poe' (Edgar Allan Poe).
'covering' means one lot of letters goes inside another.
'bananas' becomes 'mad' (slang for insane).
'poe' going around 'mad' is 'POMADE'.
'in' is the link.
(Other definitions for pomade that I've seen before include "Hair lotion" , "Perfumed hair oil" , "Perfumed hair ointment" , "Perfumed hair dressing" , "Scented ointment for the scalp" .)