Left-wing cause backed as required (2,5)
I believe the answer is:
to order
'as required' is the definition.
(I've seen this before)
'left-wing cause backed' is the wordplay.
'left wing' becomes 'red' (red can informally mean communist).
'cause' becomes 'root' (as in a root cause).
'backed' says the letters should be written in reverse.
'red'+'root'='redroot'
'redroot' in reverse letter order is '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" .)