Open boxes are behind electronic stuff (7)
I believe the answer is:
overeat
'stuff' is the definition.
(synonyms)
'open boxes are behind electronic' is the wordplay.
'open' becomes 'overt' (synonyms).
'boxes' indicates putting letters inside (the outside letters must 'box' inside letters in).
'are' becomes 'a' (short for 'are', historical unit of measurement).
'behind' means one lot of letters go next to another.
'electronic' becomes 'e' (as in e-mail or e-books).
'a' after 'e' is 'ea'.
'overt' placed around 'ea' is 'OVEREAT'.
(Other definitions for overeat that I've seen before include "Gorge oneself" , "Gobble to excess" , "Consume too much food, stuff your stomach" , "Take too much food" , "Binge" .)