Make boxes millions burn (7)
I believe the answer is:
cremate
'burn' is the definition.
(cremating is a kind of burning)
'make boxes millions' is the wordplay.
'make' becomes 'create' ('create' can be a synonym of 'make'**).
'boxes' indicates putting letters inside (the outside letters must 'box' inside letters in).
'millions' becomes 'm' (abbreviation).
'create' placed around 'm' is 'CREMATE'.
(Other definitions for cremate that I've seen before include "Burn (a dead body)" , "Have a burn-up" , "Burn a body to ashes" , "Burn a corpse to ashes" , "Destroy body by burning" .)