Stone work filling in wall (4)
I believe the answer is:
opal
'stone' is the definition.
(gemstone)
'work filling in wall' is the wordplay.
'work' becomes 'op' (common abbreviation for 'opus').
'filling in' says to take the centre (the content or filling of the word).
The middle of 'wall' is 'al'.
'op'+'al'='OPAL'
(Other definitions for opal that I've seen before include "Delicately coloured gem" , "Precious stuff" , "Milky iridescent stone" , "Gem with delicate colours" , "precious jewel" .)