Old pawnbroker shedding pound in weight (5)
I believe the answer is:
ounce
'weight' is the definition.
(ounce is a kind of weight)
'old pawnbroker shedding pound' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'pawnbroker' becomes 'uncle' (old-fashioned term for a pawnbroker).
'shedding' is a deletion indicator.
'pound' becomes 'l'.
'uncle' with 'l' taken out is 'unce'.
'o'+'unce'='OUNCE'
'in' is the link.
(Other definitions for ounce that I've seen before include "Unit of weight; cat" , "Small quantity - weight" , "Small quantity - snow leopard" , "Large cat" , "one of sixteen in a pound" .)