Difficult thing from High Command (4,5)
I believe the answer is:
tall order
'difficult thing' is the definition.
The definition and answer can be both acts as well as being singular nouns.
Perhaps there's a link between them I don't understand?
'high command' is the wordplay.
'high' becomes 'tall' (similar in meaning).
'command' becomes 'order' (order is a kind of command).
'tall'+'order'='TALL ORDER'
'from' acts as a link.
Can you help me to learn more?
(Other definitions for tall order that I've seen before include "hard to fulfil?" , "Unreasonable demand" , "unreasonable request" , "Lofty command - that's a difficult task" , "for brotherhood of high standing" .)