High Command demanding assignment? (4,5)
I believe the answer is:
tall order
'assignment?' is the definition.
(I have seen 'Difficult assignment' mean 'tall order' so perhaps 'assignment' could also mean 'tall order')
'high command' is the wordplay.
'high' becomes 'tall' (similar in meaning).
'command' becomes 'order' (order is a kind of command).
'tall'+'order'='TALL ORDER'
'demanding' is the link.
(Other definitions for tall order that I've seen before include "Challenge" , "Request to tax" , "delivery of sequoias?" , "demanding request" , "as the building of Babel was?" .)