A pineapple juice carton lasts during class (7)
I believe the answer is:
grenade
'a pineapple' is the definition.
(pineapple can informally mean a grenade)
'juice carton lasts during class' is the wordplay.
'lasts' indicates one should take the final letters (I've seen 'last of' mean this).
'during' means one lot of letters goes inside another.
'class' becomes 'grade' (synonyms).
The final letters of 'juice carton' are 'en'.
'en' inserted within 'grade' is 'GRENADE'.
(Other definitions for grenade that I've seen before include "ultimate weapon" , "Small explosive device" , "Hand-thrown bomb" , "Angered (anag)" , "Handy bomb" .)