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 may be the basis of the clue (or it may be nonsense).
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" .)