Old Conservative carves up wall-coating (6)
I believe the answer is:
stucco
'wall-coating' is the definition.
(type of plaster)
'old conservative carves up' is the wordplay.
'old' becomes 'o' (abbreviation as in OT - Old Testament).
'conservative' becomes 'C' (UK political abbreviation).
'carves' becomes 'cuts' (I can't explain this - if you can you should give a lot more credence to this answer).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'o'+'c'+'cuts'='occuts'
'occuts' back-to-front is 'STUCCO'.
Can you help me to learn more?
(Other definitions for stucco that I've seen before include "Fine plaster used in cornices, mouldings" , "Kind of plaster moulded into decorations" , "Plaster used to coat walls" , "material for covering wall" , "Plaster for decorative work" .)