Burden is nought when carried by boy (4)
I believe the answer is:
load
'burden' is the definition.
(I know that burden can be written as load)
'nought when carried by boy' is the wordplay.
'nought' becomes 'O' (O resembles 0).
'when carried by' indicates putting letters inside.
'boy' becomes 'lad' (informal term for boy).
'o' placed into 'lad' is 'LOAD'.
'is' is the link.
(Other definitions for load that I've seen before include "Cargo - burden" , "Charge a gun" , "Weight, source of pressure" , "Burden, onus" , "pack" .)