Some bananas' genetic material hard to reverse (4)
I believe the answer is:
hand
'some bananas'' is the definition.
(hand can refer to a bunch of bananas)
'genetic material hard to reverse' is the wordplay.
'genetic material' becomes 'DNA' (deoxyribonucleic acid).
'hard' becomes 'h' (abbreviation used in pencil classifications).
'to reverse' says the letters should be written in reverse.
'dna'+'h'='dnah'
'dnah' back-to-front is 'HAND'.
(Other definitions for hand that I've seen before include "Clock-pointer" , "Mitt" , "Script" , "Limb extremity" , "See 12" .)