Pineapple and some bananas eaten (not “ate”) in class (4,7)
I believe the answer is:
hand grenade
'eaten not ate in class' is the definition.
I can't judge whether this definition defines the answer.
'pineapple and some bananas' is the wordplay.
'pineapple' becomes 'grenade' (I've seen this before).
'and' means one lot of letters go next to another (I've seen this in other clues).
'some bananas' becomes 'hand' (I've seen this before).
'grenade' after 'hand' is 'HAND-GRENADE'.
(Other definitions for hand grenade that I've seen before include "Explosive device that is thrown" , "Military explosive" , "pineapple" , "Manually launched explosive device" , "Tree with edible nuts" .)