Wretched rag? No! A valued wool (6)
I believe the answer is:
angora
'valued wool' is the definition.
'angora' can be an answer for 'wool' (I've seen this before). I am not sure about the 'valued' bit.
'wretched rag? no a' is the wordplay.
'wretched' indicates anagramming the letters.
'rag'+'no'='ragno'
'ragno' with letters rearranged gives 'angor'.
'angor'+'a'='ANGORA'
Can you help me to learn more?
(Other definitions for angora that I've seen before include "Name of goat yielding 10 across" , "Goat producing mohair" , "Long-haired cat, goat, or rabbit" , "Mohair wool" , "Wool from hair of goats" .)