Curtains put up across mantle regularly pulled apart (9)
I believe the answer is:
separated
'pulled apart' is the definition.
('separate' can be a synonym of 'pull apart')
'curtains put up across mantle regularly' is the wordplay.
'curtains' becomes 'drapes' ('drape' can be a synonym of 'curtain'**).
'put up' says the letters should be written in reverse.
'across' is an insertion indicator.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
The alternating letters of 'mantle' are 'ate'.
'drapes' in reverse letter order is 'separd'.
'separd' enclosing 'ate' is 'SEPARATED'.
(Other definitions for separated that I've seen before include "Broke up" , "Distinguished -- divided" , "Split" , "in different cages?" , "No longer connected or joined" .)