Weight not possibly over two-fifths of a stone? (5)
I believe the answer is:
tonne
'weight' is the definition.
(I've seen this before)
'not possibly over two-fifths of a stone?' is the wordplay.
I cannot really see how this works, but
an anagram of 'not' is 'ton' which is located in the answer.
The remaining letters 'ne' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for tonne that I've seen before include "One thousand kilograms" , "Heavyweight" , "Great weight" , "Metric weight unit" , "1,000 kg" .)