Scarce piece is usually Welsh (7)
I believe the answer is:
rarebit
'usually welsh' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'scarce piece' is the wordplay.
'scarce' becomes 'rare' (similar in meaning).
'piece' becomes 'bit' (synonyms).
'rare'+'bit'='RAREBIT'
'is' is the link.
Can you help me to learn more?
(Other definitions for rarebit that I've seen before include "Welsh snack" , "Cheese on toast - arbiter (anag)" , "Melted cheese on hot toast" , "Cheesy snack" , "Cheese/toast savoury" .)