Burden? Lad carries nothing! (4)
I believe the answer is:
load
'burden?' is the definition.
(I know that burden can be written as load)
'lad carries nothing' is the wordplay.
'carries' indicates putting letters inside (inserted letters carried inside).
'nothing' becomes 'O' (looks like zero - 0).
'lad' enclosing 'o' is 'LOAD'.
(Other definitions for load that I've seen before include "Goods carried by a large vehicle" , "A heavy burden" , "Weight, source of pressure" , "that's put on commercial vehicle" , "pack" .)