Not really more than a ton (5)
I believe the answer is:
stone
'a ton' is the definition.
The answer and definition can be both quantities as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'not really more' is the wordplay.
I cannot quite understand how this works, but
an anagram of 'not' is 'ton' which is located in the answer.
The remaining letters 'se' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'than' is the link.
Can you help me to learn more?
(Other definitions for stone that I've seen before include "Hard lump of mineral matter" , "Imperial unit of weight" , "Olive pip" , "Rock; gem" , "Kernel" .)