Old pawnbroker sheds pound in weight (5)
I believe the answer is:
ounce
'pound in weight' is the definition.
'ounce' can be an answer for 'weight' (ounce is a kind of weight). I'm unsure of the 'pound in' bit.
'old pawnbroker sheds' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'pawnbroker sheds' becomes 'unce' (I can't justify this - if you can you should believe this answer much more).
'o'+'unce'='OUNCE'
Can you help me to learn more?
(Other definitions for ounce that I've seen before include "Panther, perhaps" , "Weight; big cat" , "one of a number in the pound" , "On cue (anag.)" , "Carnivore" .)