Boy gets round cargo (4)
I believe the answer is:
load
'cargo' is the definition.
('load' can be a synonym of 'cargo')
'boy gets round' is the wordplay.
'boy' becomes 'lad' (lad is a kind of boy**).
'gets' indicates putting letters inside (gets can mean captures or absorbs).
'round' becomes 'o' (round can mean a round object).
'lad' going around 'o' is 'LOAD'.
(Other definitions for load that I've seen before include "Burden, onus" , "Weight to be transported" , "A heavy burden" , "Weigh down" , "that's put on commercial vehicle" .)