Slave back in shifting coal out of doors (8)
I believe the answer is:
alfresco
'out of doors' is the definition.
('alfresco' can be a synonym of 'out of doors')
'slave back in shifting coal' is the wordplay.
'slave' becomes 'serf' (I've seen this before).
'back' is a reversal indicator.
'in' indicates putting letters inside.
'shifting' indicates an anagram.
'serf' in reverse letter order is 'fres'.
'coal' is an anagram of 'alco'.
'fres' placed inside 'alco' is 'ALFRESCO'.
(Other definitions for alfresco that I've seen before include "Open-air" , "In the open air" , "where it's cool" , "Located outdoors" , "Outside" .)