Boy keeps old pack (4)
I believe the answer is:
load
'pack' is the definition.
(loading is a kind of packing)
'boy keeps old' is the wordplay.
'boy' becomes 'lad' (informal term for boy).
'keeps' means one lot of letters goes inside another (inserted letters are kept inside).
'old' becomes 'o' (abbreviation).
'lad' going around 'o' is 'LOAD'.
(Other definitions for load that I've seen before include "Weight to be transported" , "that's put on commercial vehicle" , "Weigh down" , "A heavy burden" , "Charge a gun" .)