Easy progress, but suffering on grasslands (5,7)
I believe the answer is:
plain sailing
'easy progress' is the definition.
(I've seen this before)
'suffering on grasslands' is the wordplay.
'suffering' becomes 'ailing' (ailing is a kind of suffering**).
'on' is a charade indicator (letters next to each other).
'grasslands' becomes 'plains' (I've seen this before).
'ailing' after 'plains' is 'PLAIN SAILING'.
'but' is the link.
(Other definitions for plain sailing that I've seen before include "Problem-free progress" , "Easy and unobstructed progress" , "Simple" , "Trouble-free progress" , "Progress without obstruction" .)