Stone wall's bare, needing work on face (4)
I believe the answer is:
opal
'stone' is the definition.
(gemstone)
'wall's bare needing work on face' is the wordplay.
I cannot really understand how this works, but
'work' could be 'op' (abbreviation for opus) and 'op' is present in the answer.
The remaining letters 'al' 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 opal that I've seen before include "Milky white gemstone" , "One in a ring perhaps" , "Milky-white gemstone with flashes of colour" , "little jewel" , "Semi-precious stone of milky or bluish colour" .)