Three bananas, round or straight? (6)
I believe the answer is:
hetero
'straight?' is the definition.
(short for heterosexual)
'three bananas round' is the wordplay.
'bananas' indicates anagramming the letters (informal term for mad).
'round' becomes 'o' (round can mean a round object).
'three' with letters rearranged gives 'heter'.
'heter'+'o'='HETERO'
'or' acts as a link.
(Other definitions for hetero that I've seen before include "Straight (abbr)" , "Person attracted to the opposite sex (abbr)" , "Prefix signifying other" .)