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' is an insertion indicator (inserted letters are kept inside).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'lad' placed 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" .)