Burden or weight (4)
I believe the answer is:
load
I believe this clue is a double definition.
'burden' is the first definition.
(synonyms)
'weight' is the second definition.
(load is a kind of weight)
(Other definitions for load that I've seen before include "Burden, onus" , "Cargo - burden" , "A heavy burden" , "Amount" , "Goods carried by a large vehicle" .)