A tough assignment from high command (4,5)
I believe the answer is:
tall order
'a tough assignment' is the definition.
(I've seen this in another clue)
'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' is the link.
(Other definitions for tall order that I've seen before include "Unreasonable demand" , "Difficult assignment" , "Demand to tax" , "unreasonable request" , "not easily satisfied" .)