Marie's dog, that is (5)
I believe the answer is:
curie
'marie's' is the definition.
Both the answer and definition are singular nouns.
Perhaps there's a link between them I don't understand?
'dog that is' is the wordplay.
'dog' becomes 'cur' (cur is a kind of dog).
'that is' becomes 'ie' (synonyms).
'cur'+'ie'='CURIE'
Can you help me to learn more?
(Other definitions for curie that I've seen before include "One of two Nobel prizewinners" , "measure of radioactivity" , "Some radiation" , "Scientist" , "Nobel Prize winner for physics (1903) and chemistry (1911), d. 1934" .)