This made-up tally of Corinth, say (7)
I believe the answer is:
isthmus
'corinth say' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'this made-up tally' is the wordplay.
'made' indicates an anagram.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'tally' becomes 'sum' (synonyms).
'sum' in reverse letter order is 'mus'.
'this' anagrammed gives 'isth'.
'isth'+'mus'='ISTHMUS'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for isthmus that I've seen before include "Such a narrow area" , "Narrow strip of land connecting two larger land masses" , "Tongue" , "Neck of land connecting larger areas" , "This sum for a narrow piece of land" .)