Boy has nothing to pack (4)
I believe the answer is:
load
'to pack' is the definition.
(loading is a kind of packing)
'boy has nothing' is the wordplay.
'boy' becomes 'lad' (informal term for boy).
'has' means one lot of letters goes inside another.
'nothing' becomes 'O' (looks like zero - 0).
'lad' enclosing 'o' is 'LOAD'.
(Other definitions for load that I've seen before include "Great quantity" , "Amount" , "Charge a gun" , "A heavy burden" , "Weight to be transported" .)