Good lord, that is Agatha! (8)
I believe the answer is:
christie
'agatha' is the definition.
Both the answer and definition are singular nouns.
Maybe there's an association between them I don't understand?
'good lord that is' is the wordplay.
'good lord' becomes 'christ' (Christ is an example. I am not sure about the 'good' bit.).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'christ'+'ie'='CHRISTIE'
Can you help me to learn more?
(Other definitions for christie that I've seen before include "Agatha ..., mystery story writer" , "source of many mysteries" , "family with lots to do?" , "Serial killer" , "our 1/4 Across [GOLD MEDALLIST] in 1992" .)