Communist cause overturned on demand (2,5)
I believe the answer is:
to order
'on demand' is the definition.
(available to order)
'communist cause overturned' is the wordplay.
'communist' becomes 'red' (red can informally mean communist).
'cause' becomes 'root' (as in a root cause).
'overturned' says the letters should be written backwards.
'red'+'root'='redroot'
'redroot' reversed gives 'TO ORDER'.
(Other definitions for to order that I've seen before include "In fulfilment of an instruction" , "as required" , "In accordance with specification" , "As specified" .)