Showed me dad's rotten bananas (12)
I believe the answer is:
demonstrated
'showed' is the definition.
('demonstrate' can be a synonym of 'show')
'me dad's rotten bananas' is the wordplay.
'bananas' indicates anagramming the letters (informal term for mad).
'me'+'dads'+'rotten'='medadsrotten'
'medadsrotten' anagrammed gives 'DEMONSTRATED'.
(Other definitions for demonstrated that I've seen before include "shown up" , "made public protest" .)