Aussie crook on prairies makes easy progress (5,7)
I believe the answer is:
plain sailing
'easy progress' is the definition.
(I've seen this before)
'aussie crook on prairies' is the wordplay.
'aussie crook' becomes 'ailing' (I can't justify this - if you can you should believe this answer much more).
'on' says to put letters next to each other.
'prairies' becomes 'plains' (I've seen this before).
'ailing' after 'plains' is 'PLAIN-SAILING'.
'makes' is the link.
Can you help me to learn more?
(Other definitions for plain sailing that I've seen before include "No problem (for yachtsman?)" , "Problem-free progress" , "progress made in prairie schooner?" , "Progress without obstruction" , "Trouble-free progress" .)