Finally reduced world records (5)
I believe the answer is:
atlas
'world records' is the definition.
Although both the answer and definition are nouns, they are not in the same form of the noun.
'finally reduced' is the wordplay.
'finally' becomes 'at last' (synonyms).
'reduced' means to remove the last letter.
'atlast' with its last letter taken off is 'ATLAS'.
Can you help me to learn more?
(Other definitions for atlas that I've seen before include "collected world records?" , "Album of maps" , "One of the Titans" , "Geographical book" , "It appears at the top of the spine" .)