Old relative losing large amount of weight (5)

I believe the answer is:
ounce
'weight' is the definition.
(unit of weight)
'old relative losing large amount' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'relative losing large amount' becomes 'unce' (I can't explain this - if you can you should give a lot more credence to this answer).
'o'+'unce'='OUNCE'
'of' is the link.
Can you help me to learn more?
(Other definitions for ounce that I've seen before include "Approx. twenty-eight gm" , "little weight" , "Small unit of weight" , "Panther, perhaps" , "prowler" .)
