Shop owner’s ending with a huge amount (3,2)
I believe the answer is:
rat on
'shop' is the definition.
(I've seen this before)
'owner's ending with a huge amount' is the wordplay.
'ending' says to take the final letters.
'with' says to put letters next to each other.
'huge amount' becomes 'ton'.
The last letter of 'owner' is 'r'.
'r'+'a'+'ton'='RAT ON'
(Other definitions for rat on that I've seen before include "shop" , "desert" , "Give away" , "Betray" , "grass up" .)