Not too good east of prairies for a picnic (5,7)
I believe the answer is:
plain sailing
'a picnic' is the definition.
'not too good east of prairies' is the wordplay.
'not too good' becomes 'ailing'.
'east of' says to put letters next to each other.
'prairies' becomes 'plains' (I've seen this before).
'ailing' after 'plains' is 'PLAIN SAILING'.
'for' acts as a link.
(Other definitions for plain sailing that I've seen before include "Trouble-free progress" , "Simple" , "Progress without obstruction" , "A breeze" , "No problem (for yachtsman?)" .)