Boy finds nothing in pack (4)
I believe the answer is:
load
'pack' is the definition.
(I know that pack is a kind of load)
'boy finds nothing in' is the wordplay.
'boy' becomes 'lad' (informal term for boy).
'finds nothing' becomes 'o' (I can't explain this - if you can you should believe this answer much more).
'in' indicates putting letters inside.
'lad' going around 'o' is 'LOAD'.
Can you help me to learn more?
(Other definitions for load that I've seen before include "Goods carried by a large vehicle" , "Weight, source of pressure" , "Cargo - burden" , "Great quantity" , "Amount" .)