Of its kind, a crushingly big thing (5)
I believe the answer is:
molar
'of' is the definition.
Both the answer and definition are singular nouns.
Maybe you can see an association between them that I don't see?
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for molar that I've seen before include "Type of tooth" , "Large tooth" , "of ivory, perhaps" , "that could be capped" , "Grinding tooth in the back of a mammal's mouth" .)