Boy keeps old pack (4)
I believe the answer is:
load
'pack' is the definition.
(I've seen this before)
'boy keeps old' is the wordplay.
'boy' becomes 'lad' (lad is a kind of boy**).
'keeps' indicates putting letters inside.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'lad' enclosing '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" .)