Difficult assignment from High Command (4,5)
I believe the answer is:
tall order
'difficult assignment' is the definition.
(I've seen this before)
'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.
(Other definitions for tall order that I've seen before include "Formidable task" , "Demand to tax" , "as the building of Babel was?" , "Request to tax" , "not easily satisfied" .)