Boy ate doughnut pack (4)
I believe the answer is:
load
'pack' is the definition.
(I know that pack is a more specific form of the action load)
'boy ate doughnut' is the wordplay.
'boy' becomes 'lad' (informal term for boy).
'ate' means one lot of letters goes inside another.
'doughnut' becomes 'o' (letter resembling a doughnut).
'lad' placed around 'o' is 'LOAD'.
(Other definitions for load that I've seen before include "pack" , "Weigh down" , "Charge a gun" , "Goods carried by a large vehicle" , "Weight, source of pressure" .)