Remarkable stable state that is difficult to achieve (4,5)
I believe the answer is:
tall order
'is difficult to achieve' is the definition.
I can't tell whether this definition defines the answer.
'remarkable stable state' is the wordplay.
'remarkable' becomes 'tall' (I've seen this before).
'stable state' becomes 'order' (order is a kind of state. I am not sure about the 'stable' bit.).
'tall'+'order'='TALL ORDER'
'that' is the link.
Can you help me to learn more?
(Other definitions for tall order that I've seen before include "Difficult demand" , "Demanding work" , "Lot to ask" , "unreasonable request" , "as the building of Babel was?" .)