Burden is nothing when carried by boy (4)
I believe the answer is:
load
'burden' is the definition.
('load' can be a synonym of 'burden')
'nothing when carried by boy' is the wordplay.
'nothing' becomes 'o' (looks like zero - 0).
'when carried by' is an insertion indicator.
'boy' becomes 'lad' (lad is a kind of boy**).
'o' inserted within 'lad' is 'LOAD'.
'is' acts as a link.
(Other definitions for load that I've seen before include "Burden, onus" , "Charge a gun" , "Goods carried by a large vehicle" , "A heavy burden" , "pack" .)