A difficult task for High Command (4,5)
I believe the answer is:
tall order
'a difficult task' is the definition.
(I've seen this before)
'high command' is the wordplay.
'high' becomes 'tall' ('tall' is associated in meaning with 'high'**).
'command' becomes 'order' (order is a kind of command).
'tall'+'order'='TALL ORDER'
'for' is the link.
(Other definitions for tall order that I've seen before include "I want a skyscraper built by tomorrow" , "Challenge" , "Unreasonable demand" , "as the building of Babel was?" , "Demand to tax" .)