Huge object to keep down (10)
I believe the answer is:
thundering
'huge' is the definition.
Both the answer and definition are adjectives. Perhaps you can see a link between them that I can't see?
'object to keep down' is the wordplay.
'object' becomes 'thing' (an object is a thing or item).
'to keep' means one lot of letters goes inside another.
'down' becomes 'under' (under means further down).
'thing' enclosing 'under' is 'THUNDERING'.
Can you help me to learn more?
(Other definitions for thundering that I've seen before include "Making a loud noise" , "Tremendous" .)