Not large, just ten stone (4)
I believe the answer is:
onyx
'not large just ten stone' is the definition.
'onyx' can be an answer for 'stone' (thesaurus). I'm not sure about the 'not large just ten' bit.
'not large just ten stone' is the wordplay.
'not' indicates named letters should be taken away.
'large' becomes 'l'.
'just' becomes 'only' ('only' can be a synonym of 'just').
'ten stone' becomes 'x' (I can't justify this - if you can you should believe this answer much more).
'only' with 'l' taken out is 'ony'.
'ony'+'x'='ONYX'
Can you help me to learn more?
(Other definitions for onyx that I've seen before include "Semi-precious stone used in making cameos" , "Agate with different bands of colour" , "Banded chalcedony" , "Banded gemstone" , "Variety of agate, used as a gemstone" .)