Old relative sheds pounds? A lot less than that (5)
I believe the answer is:
ounce
'a lot less than that' is the definition.
The definition and answer can be both quantities as well as being singular nouns.
Maybe you can see an association between them that I can't see?
'old relative sheds pounds?' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'relative sheds pounds?' becomes 'unce' (I am not sure about this - if you are sure 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 "28.3495 grammes" , "Lightweight" , "Wildcat" , "Enough to tip the scales" , "Oz" .)