Sloppy Welsh rarebit, Shaw rejected, is extraordinarily bad (8)
I believe the answer is:
terrible
'extraordinarily bad' is the definition.
'terrible' can be an answer for 'bad' (similar in meaning). I am unsure of the 'extraordinarily' bit.
'sloppy welsh rarebit shaw rejected' is the wordplay.
'sloppy' indicates an anagram.
'rejected' is a deletion indicator.
'welsh'+'rarebit'='welshrarebit'
'welshrarebit' with 'shaw' taken away is 'elrrebit'.
'elrrebit' is an anagram of 'TERRIBLE'.
'is' is the link.
Can you help me to learn more?
(Other definitions for terrible that I've seen before include "What precedes twos < and follows child in France" , "Awful, dire" , "Awful, dreadful" , "Really bad" , "Atrocious" .)