It describes a lot - somehow her mum heard ten (5,3,6)
'it describes a lot' is the definition.
(a lot in an auction goes under the hammer)
'somehow her mum heard ten' is the wordplay.
'somehow' indicates an anagram.
'hermumheardten' anagrammed gives 'UNDER THE HAMMER'.
(Other definitions for under the hammer that I've seen before include "a lot?", "Lots come here", "Ensure your thumb is not", "For sale by auction", "on the block".)