She isn't quite worthless (5)
I believe the answer is:
penny
'worthless' is the definition.
Both the definition and answer are adjectives. Perhaps there's an association between them I don't understand?
'she isn't quite' is the wordplay.
'she' becomes 'Pen'.
'isn't quite' becomes 'ny' (I can't justify this - if you can you should give a lot more credence to this answer).
'pen'+'ny'='PENNY'
Can you help me to learn more?
(Other definitions for penny that I've seen before include "Denarius" , "Little bit of 18" , "Coin spent by the desperate?" , "Bronze coin" , "Lancaster" .)