Top Shop’s offering clobber (smaller), with every exchange of small and large (10)
I believe the answer is:
bestseller
'clobber smaller with every exchange of small and large' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.
'top shop's offering' is the wordplay.
'top' becomes 'best' (associated in meaning).
'shop's offering' becomes 'seller' (I can't justify this - if you can you should believe this answer much more).
'best'+'seller'='BESTSELLER'
Can you help me to learn more?
(Other definitions for bestseller that I've seen before include "top author" , "Dickens for instance" .)